Move js_ast library to its own package under pkg

pkg/js_ast/lib/src/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_ast;
 
 
@@ skipping 1316 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);
   }
 }