dart2js cps: Streamline expressions and primitives.

pkg/compiler/lib/src/cps_ir/mutable_ssa.dart

Index: pkg/compiler/lib/src/cps_ir/mutable_ssa.dart
diff --git a/pkg/compiler/lib/src/cps_ir/mutable_ssa.dart b/pkg/compiler/lib/src/cps_ir/mutable_ssa.dart
index abf85cd611c1bec8c1aaf9436bd15871af01e08a..690172b8ff8a915846590a9b4d6c771041c199c2 100644
--- a/pkg/compiler/lib/src/cps_ir/mutable_ssa.dart
+++ b/pkg/compiler/lib/src/cps_ir/mutable_ssa.dart
@@ -155,15 +155,17 @@ class MutableVariableEliminator implements Pass {
         // Remove the mutable variable binding.
         node.value.unlink();
         removeNode(node);
-      } else if (node is SetMutableVariable &&
-                 shouldRewrite(node.variable.definition)) {
-        // As above, update the environment, preserve variables and remove
-        // the mutable variable assignment.
-        MutableVariable variable = node.variable.definition;
-        environment[variable] = node.value.definition;
-        mergeHints(variable, node.value.definition);
-        node.value.unlink();
-        removeNode(node);
+      } else if (node is LetPrim && node.primitive is SetMutableVariable) {
+        SetMutableVariable setter = node.primitive;
+        MutableVariable variable = setter.variable.definition;
+        if (shouldRewrite(variable)) {
+          // As above, update the environment, preserve variables and remove
+          // the mutable variable assignment.
+          environment[variable] = setter.value.definition;
+          mergeHints(variable, setter.value.definition);
+          setter.value.unlink();
+          removeNode(node);
+        }
       } else if (node is LetPrim && node.primitive is GetMutableVariable) {
         GetMutableVariable getter = node.primitive;
         MutableVariable variable = getter.variable.definition;