Prevent leaks from caching compiler.

dart/pkg/dart2js_incremental/lib/caching_compiler.dart

Index: dart/pkg/dart2js_incremental/lib/caching_compiler.dart
diff --git a/dart/pkg/dart2js_incremental/lib/caching_compiler.dart b/dart/pkg/dart2js_incremental/lib/caching_compiler.dart
index ab82e818bac05c428c163a16778093304c5ea974..38edf9791ba06f2df83f987087a3e403ffe06fae 100644
--- a/dart/pkg/dart2js_incremental/lib/caching_compiler.dart
+++ b/dart/pkg/dart2js_incremental/lib/caching_compiler.dart
@@ -85,8 +85,12 @@ Future<Compiler> reuseCompiler(
         ..compilationFailed = false;
     JavaScriptBackend backend = compiler.backend;
 
-    backend.emitter.oldEmitter.cachedElements.addAll(
-        backend.generatedCode.keys);
+    // TODO(ahe): Seems this cache only serves to tell
+    // [OldEmitter.invalidateCaches] if it was invoked on a full compile (in
+    // which case nothing should be invalidated), or if it is an incremental
+    // compilation (in which case, holders/owners of newly compiled methods
+    // must be invalidated).
+    backend.emitter.oldEmitter.cachedElements.add(null);
 
     compiler.enqueuer.codegen.newlyEnqueuedElements.clear();