Rewrite for-in loop as indexing loop for JavaScript indexable arrays

pkg/compiler/lib/src/ssa/builder.dart

Index: pkg/compiler/lib/src/ssa/builder.dart
diff --git a/pkg/compiler/lib/src/ssa/builder.dart b/pkg/compiler/lib/src/ssa/builder.dart
index d5e4c29d6953f6f8c23e4bef9cbf64bced6a001b..8d1ef6989ee6712db21b9c8e060b8b32e2c74818 100644
--- a/pkg/compiler/lib/src/ssa/builder.dart
+++ b/pkg/compiler/lib/src/ssa/builder.dart
@@ -5630,6 +5630,7 @@ class SsaBuilder extends NewResolvedVisitor {
       //
       //     array.length == _end || throwConcurrentModificationError(array)
       //
+      /*
       SsaBranchBuilder branchBuilder = new SsaBranchBuilder(this, node);
       branchBuilder.handleLogicalAndOr(
           () {
@@ -5643,6 +5644,13 @@ class SsaBuilder extends NewResolvedVisitor {
           },
           isAnd: false);
       pop();
+      */
+      HInstruction length = buildGetLength();
+      push(new HIdentity(length, originalLength, null, boolType));
+      pushInvokeStatic(node,
+          backend.getThrowConcurrentModificationError(),
+          [pop(), array]);
+      pop();
     }
 
     void buildInitializer() {
@@ -5651,9 +5659,9 @@ class SsaBuilder extends NewResolvedVisitor {
       isFixed = isFixedLength(array.instructionType, compiler);
       localsHandler.updateLocal(indexVariable,
           graph.addConstantInt(0, compiler));
-      if (!isFixed) {
+      //if (!isFixed) {

[sra1, 2015/04/16 19:27:09]

This will be changed back, was just for debugging lack of optimization of the
check for trivial loops due to redundant phi-knots.

         originalLength = buildGetLength();
-      }
+      //}
     }
 
     HInstruction buildCondition() {
@@ -5673,7 +5681,7 @@ class SsaBuilder extends NewResolvedVisitor {
       //
       // For now we will put the check in the body, which will miss a
       // modification on the last iteration.
-      buildConcurrentModificationErrorCheck();
+      //buildConcurrentModificationErrorCheck();
 
       // Find a type for the element.
       // Use the element type of the indexer into the array.
@@ -5713,6 +5721,7 @@ class SsaBuilder extends NewResolvedVisitor {
       visit(node.body);
     }
     void buildUpdate() {
+      buildConcurrentModificationErrorCheck();
       // TODO(sra): It would be slightly shorter to generate `a[i++]` in the
       // body (and that more closely follows what an inlined iterator would do)
       // but the code is horrible as `i+1` is carried around the loop in an