diff --git a/user/src/com/google/gwt/core/Core.gwt.xml b/user/src/com/google/gwt/core/Core.gwt.xml
index 67ab57f..cff43a5 100644
--- a/user/src/com/google/gwt/core/Core.gwt.xml
+++ b/user/src/com/google/gwt/core/Core.gwt.xml
@@ -83,6 +83,17 @@
   <define-property name="jre.logging.simpleConsoleHandler" values="ENABLED, DISABLED" />
   <set-property name="jre.logging.simpleConsoleHandler" value="ENABLED"/>
 
+  <!--
+   Configures if the UncaughtExceptionHandler will trap errors coming from
+   listening on window.error.
+   Three different flags are:
+     - Trap window on error at all times and forward (REPORT)
+     - Only trap window.onerror if there is no current window.onerror handler (REPORT_IF_NO_HANDLER)
+     - Do not trap anything (legacy behavior) (IGNORE)
+  -->
+  <define-property name="gwt.uncaughtexceptionhander.windowonerror" values="IGNORE, REPORT, REPORT_IF_NO_HANDLER" />
+  <set-property name="gwt.uncaughtexceptionhander.windowonerror" value="IGNORE"/>
+
   <!-- To support legacy logging flags -->
   <inherits name="com.google.gwt.logging.LogImpl"/>
 </module>
diff --git a/user/src/com/google/gwt/core/client/GWT.java b/user/src/com/google/gwt/core/client/GWT.java
index 037ecaa..bdcd1ca 100644
--- a/user/src/com/google/gwt/core/client/GWT.java
+++ b/user/src/com/google/gwt/core/client/GWT.java
@@ -295,6 +295,10 @@
   public static void setUncaughtExceptionHandler(
       UncaughtExceptionHandler handler) {
     uncaughtExceptionHandler = handler;
+    // Dev mode does not do this
+    if (GWT.isScript() && handler != null) {
+      Impl.maybeInitializeWindowOnError();
+    }
   }
 
   /**
diff --git a/user/src/com/google/gwt/core/client/impl/Impl.java b/user/src/com/google/gwt/core/client/impl/Impl.java
index 7236f13..5ca81a0 100644
--- a/user/src/com/google/gwt/core/client/impl/Impl.java
+++ b/user/src/com/google/gwt/core/client/impl/Impl.java
@@ -18,6 +18,7 @@
 import com.google.gwt.core.client.Duration;
 import com.google.gwt.core.client.GWT;
 import com.google.gwt.core.client.GWT.UncaughtExceptionHandler;
+import com.google.gwt.core.client.GwtScriptOnly;
 import com.google.gwt.core.client.JavaScriptException;
 import com.google.gwt.core.client.JavaScriptObject;
 
@@ -169,7 +170,76 @@
     uncaughtExceptionHandlerForTest = handler;
   }
 
+  private static boolean onErrorInitialized;
+
+  public static void maybeInitializeWindowOnError() {
+    if ("IGNORE".equals(System.getProperty("gwt.uncaughtexceptionhander.windowonerror"))) {
+      return;
+    }
+    if (onErrorInitialized) {
+      return;
+    }
+    onErrorInitialized = true;
+    boolean alwaysReport =
+        "REPORT"
+            .equals(System.getProperty("gwt.uncaughtexceptionhander.windowonerror"));
+    registerWindowOnError(alwaysReport);
+  }
+
+  // Make sure dev mode does not try to parse the JSNI method since it contains a reference to
+  // Throwable.of which is not standard Java
+  @SuppressWarnings("deprecation")
+  @GwtScriptOnly
+  public static native void registerWindowOnError(boolean reportAlways) /*-{
+    function errorHandler(msg, url, line, column, error) {
+      // IE8, IE9, IE10, safari 9, do not have an error passed
+      if (!error) {
+        error = msg + " (" + url + ":" + line
+        // IE8 and IE9 do not have the column number
+        if (column) {
+          error += ":" + column
+        }
+        error += ")";
+      }
+
+      var throwable = @java.lang.Throwable::of(*)(error);
+      @Impl::reportWindowOnError(*)(throwable);
+    };
+
+    function addOnErrorHandler(windowRef) {
+      var origHandler = windowRef.onerror;
+      if (origHandler && !reportAlways) {
+        return;
+      }
+
+      windowRef.onerror = function() {
+        errorHandler.apply(this, arguments);
+        if (origHandler) {
+          origHandler.apply(this, arguments);
+        }
+        return false;
+      };
+    }
+
+    // Note we need to trap both window.onerror and $wnd.onerror
+    // Chrome 58 & Safari (10.1) & HtmlUnit uses $wnd.error,
+    // while FF (53) /IE (even edge) listens on window.error
+    addOnErrorHandler($wnd);
+    addOnErrorHandler(window);
+  }-*/;
+
+  private static void reportWindowOnError(Throwable e) {
+    // If the error is coming from window.onerror that we registered on, we can not report it
+    // back to the browser since we would end up being called again
+    reportUncaughtException(e, false);
+  }
+
   public static void reportUncaughtException(Throwable e) {
+    reportUncaughtException(e, true);
+  }
+
+  private static void reportUncaughtException(
+      Throwable e, boolean reportSwallowedExceptionToBrowser) {
     if (Impl.uncaughtExceptionHandlerForTest != null) {
       Impl.uncaughtExceptionHandlerForTest.onUncaughtException(e);
     }
@@ -188,7 +258,7 @@
     }
 
     // Make sure that the exception is not swallowed
-    if (GWT.isClient()) {
+    if (GWT.isClient() && reportSwallowedExceptionToBrowser) {
       reportToBrowser(e);
     } else {
       System.err.print("Uncaught exception ");
diff --git a/user/src/com/google/gwt/jsonp/client/JsonpRequest.java b/user/src/com/google/gwt/jsonp/client/JsonpRequest.java
index a90e822..4b410d4 100644
--- a/user/src/com/google/gwt/jsonp/client/JsonpRequest.java
+++ b/user/src/com/google/gwt/jsonp/client/JsonpRequest.java
@@ -329,7 +329,7 @@
           // don't want to unregister the callback since there may be pending
           // requests that have not yet come back and we don't want them to
           // have an undefined callback function.
-          unregisterCallbacks(CALLBACKS);
+          unregisterCallbacks(CALLBACKS, callbackId);
         }
         Node script = Document.get().getElementById(callbackId);
         if (script != null) {
@@ -340,7 +340,7 @@
     });
   }
 
-  private native void unregisterCallbacks(JavaScriptObject callbacks) /*-{
-    delete callbacks[this.@com.google.gwt.jsonp.client.JsonpRequest::callbackId];
+  private native void unregisterCallbacks(JavaScriptObject callbacks, String callbackId) /*-{
+    callbacks[callbackId].onSuccess = callbacks[callbackId].onFailure = function() {};
   }-*/;
 }
diff --git a/user/test/com/google/gwt/core/CoreSuite.java b/user/test/com/google/gwt/core/CoreSuite.java
index aef8f7c..49e051c 100644
--- a/user/test/com/google/gwt/core/CoreSuite.java
+++ b/user/test/com/google/gwt/core/CoreSuite.java
@@ -25,6 +25,7 @@
 import com.google.gwt.core.client.JsonUtilsTest;
 import com.google.gwt.core.client.SchedulerTest;
 import com.google.gwt.core.client.ScriptInjectorTest;
+import com.google.gwt.core.client.WindowOnErrorTest;
 import com.google.gwt.core.client.impl.ImplTest;
 import com.google.gwt.core.client.impl.SchedulerImplTest;
 import com.google.gwt.core.client.impl.StackTraceCreatorCollectorTest;
@@ -62,6 +63,7 @@
     suite.addTestSuite(StackTraceEmulTest.class);
     suite.addTestSuite(StackTraceNativeTest.class);
     suite.addTestSuite(StackTraceStripTest.class);
+    suite.addTestSuite(WindowOnErrorTest.class);
 
     // Uncomment to print native stack traces for different platforms
     // suite.addTestSuite(com.google.gwt.core.client.impl.StackTraceGenerator.class);
diff --git a/user/test/com/google/gwt/core/WindowOnError.gwt.xml b/user/test/com/google/gwt/core/WindowOnError.gwt.xml
new file mode 100644
index 0000000..0f34c54
--- /dev/null
+++ b/user/test/com/google/gwt/core/WindowOnError.gwt.xml
@@ -0,0 +1,18 @@
+<!--                                                                        -->
+<!-- Copyright 2017 Google Inc.                                             -->
+<!-- Licensed under the Apache License, Version 2.0 (the "License"); you    -->
+<!-- may not use this file except in compliance with the License. You may   -->
+<!-- may obtain a copy of the License at                                    -->
+<!--                                                                        -->
+<!-- http://www.apache.org/licenses/LICENSE-2.0                             -->
+<!--                                                                        -->
+<!-- Unless required by applicable law or agreed to in writing, software    -->
+<!-- distributed under the License is distributed on an "AS IS" BASIS,      -->
+<!-- WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or        -->
+<!-- implied. License for the specific language governing permissions and   -->
+<!-- limitations under the License.                                         -->
+
+<module>
+  <inherits name="com.google.gwt.core.Core" />
+  <set-property name="gwt.uncaughtexceptionhander.windowonerror" value="REPORT_IF_NO_HANDLER"/>
+</module>
diff --git a/user/test/com/google/gwt/core/client/WindowOnErrorTest.java b/user/test/com/google/gwt/core/client/WindowOnErrorTest.java
new file mode 100644
index 0000000..6752b84
--- /dev/null
+++ b/user/test/com/google/gwt/core/client/WindowOnErrorTest.java
@@ -0,0 +1,68 @@
+/*
+ * Copyright 2017 Google Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License"); you may not
+ * use this file except in compliance with the License. You may obtain a copy of
+ * the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+ * License for the specific language governing permissions and limitations under
+ * the License.
+ */
+package com.google.gwt.core.client;
+
+import com.google.gwt.junit.DoNotRunWith;
+import com.google.gwt.junit.Platform;
+import com.google.gwt.junit.client.GWTTestCase;
+import com.google.gwt.user.client.Timer;
+
+/** Test for window.onerror reporting to {@link UncaughtExceptionHandler}. */
+public class WindowOnErrorTest extends GWTTestCase {
+
+  private int reportedJsExceptionCount;
+
+  @Override
+  public String getModuleName() {
+    return "com.google.gwt.core.WindowOnError";
+  }
+
+  @Override
+  protected void reportUncaughtException(Throwable ex) {
+    // We need to distinguish here between the exceptions that we are deliberately creating
+    // and ones that should make the test fail.
+    // All our exceptions will contain "from_js" in their message
+    if (ex.getMessage().contains("from_js")) {
+      // Do not let the test fail
+      reportedJsExceptionCount++;
+      return;
+    }
+    super.reportUncaughtException(ex);
+  }
+
+  // Does not work in dev mode, since JNSI code for setting up window.onerror needs Throwable.of
+  // from super sourced code.
+  @DoNotRunWith({Platform.Devel})
+  public void testFailViaWindowOnError() {
+    delayTestFinish(2000);
+
+    new Timer() {
+      @Override
+      public void run() {
+        assertEquals(1, reportedJsExceptionCount);
+        finishTest();
+      }
+    }.schedule(1000);
+
+    throwInNonEntryMethod();
+  }
+
+  private native void throwInNonEntryMethod() /*-{
+    $wnd.setTimeout(function() {
+      throw new Error("from_js");
+    }, 0);
+  }-*/;
+}