Avoid generating VariableUse nodes with non-identifier names

sdk/lib/_internal/compiler/implementation/js/builder.dart

 // Copyright (c) 2013, 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.
 
 // Utilities for building JS ASTs at runtime.  Contains a builder class
 // and a parser that parses part of the language.
 
 part of js;
 
 
@@ skipping 661 matching lines @@
       return parseObjectInitializer();
     } else if (acceptCategory(LSQUARE)) {
       var values = <ArrayElement>[];
       if (!acceptCategory(RSQUARE)) {
         do {
           values.add(new ArrayElement(values.length, parseAssignment()));
         } while (acceptCategory(COMMA));
         expectCategory(RSQUARE);
       }
       return new ArrayInitializer(values.length, values);
-    } else if (last.startsWith("/")) {
+    } else if (last != null && last.startsWith("/")) {
       String regexp = getDelimited(lastPosition);
       getToken();
       String flags = lastToken;
       if (!acceptCategory(ALPHA)) flags = "";
       Expression expression = new RegExpLiteral(regexp + flags);
       return expression;
     } else if (acceptCategory(HASH)) {
       InterpolatedExpression expression =
           new InterpolatedExpression(interpolatedValues.length);
       interpolatedValues.add(expression);
@@ skipping 479 matching lines @@
   Catch parseCatch() {
     expectCategory(LPAREN);
     String identifier = lastToken;
     expectCategory(ALPHA);
     expectCategory(RPAREN);
     expectCategory(LBRACE);
     Block body = parseBlock();
     return new Catch(new VariableDeclaration(identifier), body);
   }
 }