Add bailouts to dart backend.

lib/compiler/implementation/dart_backend/backend.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+class BailoutException {
+  final String reason;
+
+  const BailoutException(this.reason);
+}
+
 class DartBackend extends Backend {
   final List<CompilerTask> tasks;
   final UnparseValidator unparseValidator;
 
   Map<Element, TreeElements> get resolvedElements() =>
       compiler.enqueuer.resolution.resolvedElements;
 
   DartBackend(Compiler compiler, [bool validateUnparse = false])
       : tasks = <CompilerTask>[],
       unparseValidator = new UnparseValidator(compiler, validateUnparse),
       super(compiler) {
     tasks.add(unparseValidator);
   }
 
   void enqueueHelpers(Enqueuer world) {
     // TODO(antonm): Implement this method, if needed.
   }
 
   String codegen(WorkItem work) { }
 
   void processNativeClasses(Enqueuer world,
                             Collection<LibraryElement> libraries) {
   }
 
   void assembleProgram() {
     resolvedElements.forEach((element, treeElements) {
       unparseValidator.check(element);
     });
-    compiler.assembledCode = '';
+
+    // TODO(antonm): Eventually bailouts will be proper errors.
+    void bailout(String reason) {
+      throw new BailoutException(reason);
+    }
+
+    try {
+      resolvedElements.forEach((element, treeElements) {
+        bailout('I can do nothing right now.');
+      });
+    } catch (BailoutException e) {
+      compiler.assembledCode = '''
+main() {
+  final bailout_reason = "${e.reason}";
+}
+''';
+    }
   }
 
   log(String message) => compiler.log('[DartBackend] $message');
 }