Was failing to clear out a cached value on refresh.

Issue: 3180


git-svn-id: https://google-web-toolkit.googlecode.com/svn/releases/1.6@4418 8db76d5a-ed1c-0410-87a9-c151d255dfc7
diff --git a/dev/core/src/com/google/gwt/dev/javac/CompilationState.java b/dev/core/src/com/google/gwt/dev/javac/CompilationState.java
index 0ad7a78..3ada290 100644
--- a/dev/core/src/com/google/gwt/dev/javac/CompilationState.java
+++ b/dev/core/src/com/google/gwt/dev/javac/CompilationState.java
@@ -261,5 +261,6 @@
     exposedUnits = Collections.unmodifiableSet(new HashSet<CompilationUnit>(
         unitMap.values()));
     exposedClassFileMap = null;
+    exposedClassFileMapBySource = null;
   }
 }