Integrate leg as a component in test.py. Also update presubmit script to run tests with leg.

dart/frog/leg/scanner/listener.dart

 // Copyright (c) 2011, 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.
 
 /**
  * A listener for parser events.
  */
 class Listener {
   int classCount = 0;
   int aliasCount = 0;
   int interfaceCount = 0;
   int libraryTagCount = 0;
   int topLevelMemberCount = 0;
   Identifier previousIdentifier = null;
   final Canceler canceler;
 
   Link<DeclarationBuilder> builders;
   Link<Element> topLevelElements;
 
   Listener(Canceler this.canceler)
     : builders = const EmptyLink<DeclarationBuilder>(),
       topLevelElements = const EmptyLink<Element>();
 
   void beginLibraryTag(Token token) {
+    canceler.cancel("Cannot handle library tags");
     libraryTagCount++;
   }
 
   void beginClass(Token token) {
     classCount++;
     push(token, buildClassElement);
   }
 
-  Element buildClassElement(DeclarationBuilder declaration) => null;
+  Element buildClassElement(DeclarationBuilder declaration) {
+    canceler.cancel("Cannot handle classes");
+  }
 
   void endClass(Token token) {
     handleDeclaration(pop(), token);
   }
 
   void beginInterface(Token token) {
     interfaceCount++;
     push(token, buildInterfaceElement);
   }
 
-  Element buildInterfaceElement(DeclarationBuilder declaration) => null;
+  Element buildInterfaceElement(DeclarationBuilder declaration) {
+    canceler.cancel("Cannot handle interfaces");
+  }
 
   void endInterface(Token token) {
     handleDeclaration(pop(), token);
   }
 
   void beginFunctionTypeAlias(Token token) {
     aliasCount++;
     push(token, buildFunctionTypeAliasElement);
   }
 
-  Element buildFunctionTypeAliasElement(DeclarationBuilder declaration) => null;
+  Element buildFunctionTypeAliasElement(DeclarationBuilder declaration) {
+    canceler.cancel("Cannot handle typedefs");
+  }
 
   void endFunctionTypeAlias(Token token) {
     handleDeclaration(pop(), token);
   }
 
   void beginTopLevelMember(Token token) {
     topLevelMemberCount++;
   }
 
   void topLevelMethod(Token token) {
     push(token, buildMethod);
     builders.head.name = previousIdentifier.source;
   }
 
   Element buildMethod(DeclarationBuilder declaration) {
     return new PartialFunctionElement(declaration.name,
                                       declaration.beginToken,
                                       declaration.endToken);
   }
 
   void topLevelField(Token token) {
     push(token, buildField);
     builders.head.name = previousIdentifier.source;
   }
 
-  Element buildField(DeclarationBuilder declaration) => null;
+  Element buildField(DeclarationBuilder declaration) {
+    canceler.cancel("Cannot handle fields");
+  }
 
   void endTopLevelMember(Token token) {
     handleDeclaration(pop(), token);
   }
 
   void beginTypeVariable(Token token) {
   }
 
   void endTypeVariable(Token token) {
   }
@@ skipping 304 matching lines @@
 
   void popExpressionContext() {
     assert(expressionContext.expressions.isEmpty());
     expressionContext = expressionContext.previous;
   }
 
   void pushStatement(Statement statement) {
     functionContext.push(statement);
   }
 }