Fix some bugs that blocked optimizations in the new IRs.

pkg/compiler/lib/src/tree_ir/tree_ir_builder.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.
 
 library tree_ir_builder;
 
 import '../dart2jslib.dart' as dart2js;
 import '../dart_types.dart';
 import '../elements/elements.dart';
 import '../cps_ir/cps_ir_nodes.dart' as cps_ir;
@@ skipping 171 matching lines @@
       returnContinuation = node.body.returnContinuation;
 
       phiTempVar = new Variable(node.element, null);
       body = visit(node.body);
     }
 
     return new ConstructorDefinition(node.element, parameters,
         body, initializers, node.localConstants, node.defaultParameterValues);
   }
 
-
+  /// Returns a list of variables corresponding to the arguments to a method
+  /// call or similar construct.
+  ///
+  /// The `readCount` for these variables will be incremented.
+  ///
+  /// The list will be typed as a list of [Expression] to allow inplace updates
+  /// on the list during the rewrite phases.
   List<Expression> translateArguments(List<cps_ir.Reference> args) {
     return new List<Expression>.generate(args.length,
          (int index) => getVariableReference(args[index]),
          growable: false);
   }
 
+  /// Returns the list of variables corresponding to the arguments to a join
+  /// continuation.
+  ///
+  /// The `readCount` of these variables will not be incremented. Instead,
+  /// [buildPhiAssignments] will handle the increment, if necessary.
   List<Variable> translatePhiArguments(List<cps_ir.Reference> args) {
     return new List<Variable>.generate(args.length,
-         (int index) => getVariableReference(args[index]));
+         (int index) => getVariable(args[index].definition),
+         growable: false);
   }
 
   Statement buildContinuationAssignment(
       cps_ir.Parameter parameter,
       Expression argument,
       Statement buildRest()) {
     Variable variable = getVariable(parameter);
     Statement assignment;
     if (variable == null) {
       assignment = new ExpressionStatement(argument, null);
@@ skipping 28 matching lines @@
       }
       List<int> list = rightHand[arg];
       if (list == null) {
         rightHand[arg] = list = <int>[];
       }
       list.add(i);
     }
 
     Statement first, current;
     void addAssignment(Variable dst, Variable src) {
+      ++src.readCount;
+      // `dst.writeCount` will be updated by the Assign constructor.
       if (first == null) {
         first = current = new Assign(dst, src, null);
       } else {
         current = current.next = new Assign(dst, src, null);
       }
     }
 
     List<Variable> assignmentSrc = new List<Variable>(parameters.length);
     List<bool> done = new List<bool>(parameters.length);
     void visitAssignment(int i) {
@@ skipping 284 matching lines @@
     // visited.
     internalError(CURRENT_ELEMENT_SPANNABLE, 'Unexpected IR node: $node.');
     return null;
   }
 
   Expression visitIsTrue(cps_ir.IsTrue node) {
     return getVariableReference(node.value);
   }
 }