Implement script tags.

dart/frog/leg/compiler.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 WorkItem {
   final Element element;
   TreeElements resolutionTree;
   Function run;
   Map<int, BailoutInfo> bailouts = null;
 
@@ skipping 199 matching lines @@
       while (!worklist.isEmpty()) {
         WorkItem work = worklist.removeLast();
         withCurrentElement(work.element, () => (work.run)(this));
       }
       enqueueInvokedInstanceMethods();
     } while (!worklist.isEmpty());
     emitter.assembleProgram();
   }
 
   TreeElements analyzeElement(Element element) {
+    assert(parser !== null);
     Node tree = parser.parse(element);
     validator.validate(tree);
     TreeElements elements = resolver.resolve(element);
     checker.check(tree, elements);
     return elements;
   }
 
   TreeElements analyze(WorkItem work) {
     work.resolutionTree = analyzeElement(work.element);
     return work.resolutionTree;
@@ skipping 106 matching lines @@
 
 class CompilerCancelledException implements Exception {
   final String reason;
   CompilerCancelledException(this.reason);
 
   String toString() {
     String banner = 'compiler cancelled';
     return (reason !== null) ? '$banner: $reason' : '$banner';
   }
 }