fix truncate call to use extension method

lib/src/codegen/js_codegen.dart

 // Copyright (c) 2015, 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.
 
 library dev_compiler.src.codegen.js_codegen;
 
 import 'dart:collection' show HashSet, HashMap, SplayTreeSet;
 
 import 'package:analyzer/analyzer.dart' hide ConstantEvaluator;
 import 'package:analyzer/src/generated/ast.dart' hide ConstantEvaluator;
@@ skipping 1996 matching lines @@
 
     if (binaryOperationIsPrimitive(leftType, rightType) ||
         rules.isStringType(leftType) && op.type == TokenType.PLUS) {
 
       // special cases where we inline the operation
       // these values are assumed to be non-null (determined by the checker)
       // TODO(jmesserly): it would be nice to just inline the method from core,
       // instead of special cases here.
       if (op.type == TokenType.TILDE_SLASH) {
         // `a ~/ b` is equivalent to `(a / b).truncate()`
-        code = '(# / #).truncate()';
+        var div = AstBuilder.binaryExpression(left, '/', right)
+          ..staticType = node.staticType;
+        return _emitSend(div, 'truncate', []);
       } else {
         // TODO(vsm): When do Dart ops not map to JS?
         code = '# $op #';
       }
       return js.call(code, [notNull(left), notNull(right)]);
     }
 
     return _emitSend(left, op.lexeme, [right]);
   }
 
@@ skipping 820 matching lines @@
 
 class _JsThisFinder extends JS.BaseVisitor {
   bool found = false;
   visitThis(JS.This node) {
     found = true;
   }
   visitNode(JS.Node node) {
     if (!found) super.visitNode(node);
   }
 }