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

pkg/js_ast/lib/src/template.dart

Index: pkg/js_ast/lib/src/template.dart
diff --git a/pkg/js_ast/lib/src/template.dart b/pkg/js_ast/lib/src/template.dart
index eb4089ad807090f3b6270d5192658c0e6933e8ad..e0f0d5bb53972c846165247c3343deaaaeb6b3bd 100644
--- a/pkg/js_ast/lib/src/template.dart
+++ b/pkg/js_ast/lib/src/template.dart
@@ -59,20 +59,29 @@ class Template {
     _compile();
   }
 
-  Template.withExpressionResult(this.ast)
+  Template.withExpressionResult(this.ast, {hasPlaceholders : false})
       : source = null, isExpression = true, forceCopy = false {
     assert(ast is Expression);
-    assert(_checkNoPlaceholders());
-    positionalArgumentCount = 0;
-    instantiator = (arguments) => ast;
+    if (hasPlaceholders) {
+      _compile();
+    } else {
+      assert(_checkNoPlaceholders());
+      positionalArgumentCount = 0;
+      instantiator = (arguments) => ast;
+    }
   }
 
-  Template.withStatementResult(this.ast)
+  Template.withStatementResult(this.ast, {hasPlaceholders : false})
       : source = null, isExpression = false, forceCopy = false {
     assert(ast is Statement);
-    assert(_checkNoPlaceholders());
-    positionalArgumentCount = 0;
-    instantiator = (arguments) => ast;
+
+    if (hasPlaceholders) {
+      _compile();
+    } else {
+      assert(_checkNoPlaceholders());
+      positionalArgumentCount = 0;
+      instantiator = (arguments) => ast;
+    }
   }
 
   bool _checkNoPlaceholders() {