Implement override checks.

dart/lib/compiler/implementation/compiler.dart

Index: dart/lib/compiler/implementation/compiler.dart
diff --git a/dart/lib/compiler/implementation/compiler.dart b/dart/lib/compiler/implementation/compiler.dart
index 6a78ae5e1f127347aa970adbc7507e675cef653c..5cb19337cb4fcc308c118fca646646309eb21ecd 100644
--- a/dart/lib/compiler/implementation/compiler.dart
+++ b/dart/lib/compiler/implementation/compiler.dart
@@ -303,6 +303,8 @@ class Compiler implements DiagnosticListener {
   static final int PHASE_RECOMPILING = 3;
   int phase;
 
+  bool compilationFailed = false;
+
   Compiler([this.tracer = const Tracer(),
             this.enableTypeAssertions = false,
             this.enableUserAssertions = false,
@@ -548,6 +550,8 @@ class Compiler implements DiagnosticListener {
     processQueue(enqueuer.resolution, main);
     log('Resolved ${enqueuer.resolution.resolvedElements.length} elements.');
 
+    if (compilationFailed) return;
+
     log('Compiling...');
     phase = PHASE_COMPILING;
     processQueue(enqueuer.codegen, main);
@@ -557,6 +561,8 @@ class Compiler implements DiagnosticListener {
     processRecompilationQueue(enqueuer.codegen);
     log('Compiled ${codegenWorld.generatedCode.length} methods.');
 
+    if (compilationFailed) return;
+
     backend.assembleProgram();
 
     checkQueues();
@@ -755,7 +761,7 @@ class Compiler implements DiagnosticListener {
     }
     SourceSpan span = spanFromNode(node);
 
-    reportDiagnostic(span, 'Warning: $message', api.Diagnostic.WARNING );
+    reportDiagnostic(span, 'Warning: $message', api.Diagnostic.WARNING);
   }
 
   reportError(Node node, var message) {
@@ -764,6 +770,14 @@ class Compiler implements DiagnosticListener {
     throw new CompilerCancelledException(message.toString());
   }
 
+  void reportMessage(SourceSpan span,
+                     Diagnostic message,
+                     api.Diagnostic kind) {
+    // TODO(ahe): The names Diagnostic and api.Diagnostic are in
+    // conflict. Fix it.
+    reportDiagnostic(span, "$message", kind);
+  }
+
   abstract void reportDiagnostic(SourceSpan span, String message,
                                  api.Diagnostic kind);