Implement try/catch in the JS backend.

pkg/compiler/lib/src/cps_ir/cps_ir_nodes.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.
 
 // IrNodes are kept in a separate library to have precise control over their
 // dependencies on other parts of the system.
 library dart2js.ir_nodes;
 
 import '../constants/expressions.dart';
 import '../constants/values.dart' as values show ConstantValue;
@@ skipping 19 matching lines @@
 }
 
 /// The base class of things that variables can refer to: primitives,
 /// continuations, function and continuation parameters, etc.
 abstract class Definition<T extends Definition<T>> extends Node {
   // The head of a linked-list of occurrences, in no particular order.
   Reference<T> firstRef;
 
   bool get hasAtMostOneUse  => firstRef == null || firstRef.next == null;
   bool get hasExactlyOneUse => firstRef != null && firstRef.next == null;
+  bool get hasNoUses => firstRef == null;
   bool get hasAtLeastOneUse => firstRef != null;
   bool get hasMultipleUses  => !hasAtMostOneUse;
 
   void substituteFor(Definition<T> other) {
-    if (other.firstRef == null) return;
+    if (other.hasNoUses) return;
     Reference<T> previous, current = other.firstRef;
     do {
       current.definition = this;
       previous = current;
       current = current.next;
     } while (current != null);
     previous.next = firstRef;
     if (firstRef != null) firstRef.previous = previous;
     firstRef = other.firstRef;
+    other.firstRef = null;
   }
 }
 
 /// An expression that cannot throw or diverge and has no side-effects.
 /// All primitives are named using the identity of the [Primitive] object.
 ///
 /// Primitives may allocate objects, this is not considered side-effect here.
 ///
 /// Although primitives may not mutate state, they may depend on state.
 abstract class Primitive extends Definition<Primitive> {
@@ skipping 1195 matching lines @@
     processReference(node.target);
   }
 
   processTypeExpression(TypeExpression node) {}
   @override
   visitTypeExpression(TypeExpression node) {
     processTypeExpression(node);
     node.arguments.forEach(processReference);
   }
 }