Makes sure to fail-fast on missing devirtualized method

Change-Id: I17b2f894cd50e55cca060ce0dcdf44dd844b112e
diff --git a/dev/core/src/com/google/gwt/dev/jjs/impl/Devirtualizer.java b/dev/core/src/com/google/gwt/dev/jjs/impl/Devirtualizer.java
index acf104c..b8e630f 100644
--- a/dev/core/src/com/google/gwt/dev/jjs/impl/Devirtualizer.java
+++ b/dev/core/src/com/google/gwt/dev/jjs/impl/Devirtualizer.java
@@ -552,8 +552,10 @@
       EnumMap<DispatchType, JMethod> dispatchToMethodByTargetType, JClassType targetDevirtualType) {
     if (possibleTargetTypes.contains(target)) {
       JMethod overridingMethod = findOverridingMethod(method, targetDevirtualType);
-      assert overridingMethod != null : method.getEnclosingType().getName() + "::" +
-          method.getName() + " not overridden by " + targetDevirtualType.getSimpleName();
+      if (overridingMethod == null) {
+        throw new AssertionError(method.getEnclosingType().getName() + "::" + method.getName()
+            + " not overridden by " + targetDevirtualType.getSimpleName());
+      }
       dispatchToMethodByTargetType.put(target,
           staticImplCreator.getOrCreateStaticImpl(program, overridingMethod));
     }