Add optional message to assert in Dart2js.

pkg/compiler/lib/src/resolution/tree_elements.dart

Index: pkg/compiler/lib/src/resolution/tree_elements.dart
diff --git a/pkg/compiler/lib/src/resolution/tree_elements.dart b/pkg/compiler/lib/src/resolution/tree_elements.dart
index 3fed5c391a443d4f9ca6872cb519cae5c4153786..94d7889cffff500a0e6ca7a036a02bd31569c696 100644
--- a/pkg/compiler/lib/src/resolution/tree_elements.dart
+++ b/pkg/compiler/lib/src/resolution/tree_elements.dart
@@ -71,7 +71,6 @@ abstract class TreeElements {
   void setCurrentTypeMask(ForIn node, TypeMask mask);
   void setConstant(Node node, ConstantExpression constant);
   ConstantExpression getConstant(Node node);
-  bool isAssert(Send send);
 
   /// Returns the [FunctionElement] defined by [node].
   FunctionElement getFunctionDefinition(FunctionExpression node);
@@ -140,7 +139,6 @@ class TreeElementMapping extends TreeElements {
   Map<VariableElement, List<Node>> _potentiallyMutatedInClosure;
   Map<Node, Map<VariableElement, List<Node>>> _accessedByClosureIn;
   Setlet<Element> _elements;
-  Setlet<Send> _asserts;
   Maplet<Send, SendStructure> _sendStructureMap;
   Setlet<DartType> _requiredTypes;
   bool containsTryStatement = false;
@@ -412,17 +410,6 @@ class TreeElementMapping extends TreeElements {
     }
   }
 
-  void setAssert(Send node) {
-    if (_asserts == null) {
-      _asserts = new Setlet<Send>();
-    }
-    _asserts.add(node);
-  }
-
-  bool isAssert(Send node) {
-    return _asserts != null && _asserts.contains(node);
-  }
-
   FunctionElement getFunctionDefinition(FunctionExpression node) {
     return this[node];
   }