dart2js cps: Add helpers for common IR manipulation.

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 3dea17c0ba40cf1418c74a096b7e34dcecd095c0..e2252c4d55eecf9c45e77f541b1c1db126ad722c 100644
--- a/pkg/compiler/lib/src/cps_ir/mutable_ssa.dart
+++ b/pkg/compiler/lib/src/cps_ir/mutable_ssa.dart
@@ -114,12 +114,6 @@ class MutableVariableEliminator implements Pass {
            cont.firstRef.parent is InvokeContinuation;
   }
 
-  void removeNode(InteriorNode node) {
-    InteriorNode parent = node.parent;
-    parent.body = node.body;
-    node.body.parent = parent;
-  }
-
   /// If some useful source information is attached to exactly one of the
   /// two definitions, the information is copied onto the other.
   void mergeHints(MutableVariable variable, Primitive value) {
@@ -139,7 +133,8 @@ class MutableVariableEliminator implements Pass {
   /// Continuations to be processed are put on the stack for later processing.
   void processBlock(Expression node,
                     Map<MutableVariable, Primitive> environment) {
-    for (; node is! TailExpression; node = node.next) {
+    Expression next = node.next;
+    for (; node is! TailExpression; node = next, next = node.next) {
       if (node is LetMutable && shouldRewrite(node.variable)) {
         // Put the new mutable variable on the stack while processing the body,
         // and pop it off again when done with the body.
@@ -155,7 +150,7 @@ class MutableVariableEliminator implements Pass {
 
         // Remove the mutable variable binding.
         node.value.unlink();
-        removeNode(node);
+        node.remove();
       } else if (node is LetPrim && node.primitive is SetMutable) {
         SetMutable setter = node.primitive;
         MutableVariable variable = setter.variable.definition;
@@ -165,7 +160,7 @@ class MutableVariableEliminator implements Pass {
           environment[variable] = setter.value.definition;
           mergeHints(variable, setter.value.definition);
           setter.value.unlink();
-          removeNode(node);
+          node.remove();
         }
       } else if (node is LetPrim && node.primitive is GetMutable) {
         GetMutable getter = node.primitive;
@@ -175,7 +170,7 @@ class MutableVariableEliminator implements Pass {
           Primitive value = environment[variable];
           value.substituteFor(getter);
           mergeHints(variable, value);
-          removeNode(node);
+          node.remove();
         }
       } else if (node is LetCont) {
         // Create phi parameters for each join continuation bound here, and put