dart2js: Construct the entire output as a single AST before printing.

pkg/js_ast/lib/src/template.dart

 // Copyright (c) 2014, 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.
 
 part of js_ast;
 
 class TemplateManager {
   Map<String, Template> expressionTemplates = new Map<String, Template>();
   Map<String, Template> statementTemplates = new Map<String, Template>();
 
@@ skipping 38 matching lines @@
   Instantiator instantiator;
 
   int positionalArgumentCount = -1;
 
   // Null, unless there are named holes.
   List<String> holeNames;
   bool get isPositional => holeNames == null;
 
   Template(this.source, this.ast,
            {this.isExpression: true, this.forceCopy: false}) {
+    assert(this.isExpression ? ast is Expression : ast is Statement);
     _compile();
   }
 
   Template.withExpressionResult(this.ast)
       : source = null, isExpression = true, forceCopy = false {
     assert(ast is Expression);
     assert(_checkNoPlaceholders());
     positionalArgumentCount = 0;
     instantiator = (arguments) => ast;
   }
@@ skipping 676 matching lines @@
     if (count != before) containsInterpolatedNode.add(node);
     return null;
   }
 
   visitInterpolatedNode(InterpolatedNode node) {
     containsInterpolatedNode.add(node);
     if (node.isNamed) holeNames.add(node.nameOrPosition);
     ++count;
   }
 }