Consider updates of locals in type promotion.

pkg/compiler/lib/src/inferrer/inferrer_visitor.dart

Index: pkg/compiler/lib/src/inferrer/inferrer_visitor.dart
diff --git a/pkg/compiler/lib/src/inferrer/inferrer_visitor.dart b/pkg/compiler/lib/src/inferrer/inferrer_visitor.dart
index c2b3e416d62296f61ef30f48b417191b9deb13b3..add3759a0779573cdb4d41b71369b74e0eaae4dd 100644
--- a/pkg/compiler/lib/src/inferrer/inferrer_visitor.dart
+++ b/pkg/compiler/lib/src/inferrer/inferrer_visitor.dart
@@ -150,6 +150,13 @@ class VariableScope<T> {
             ? null
             : new VariableScope<T>.deepCopyOf(other.parent);
 
+  VariableScope.shallowCopyOf(VariableScope<T> other)
+      : variables = other.variables == null
+            ? null
+            : new Map<Local, T>.from(other.variables),
+        block = other.block,
+        parent = other.parent;
+
   T operator [](Local variable) {
     T result;
     if (variables == null || (result = variables[variable]) == null) {
@@ -401,6 +408,16 @@ class LocalsHandler<T> {
         inferrer = other.inferrer,
         compiler = other.compiler;
 
+  LocalsHandler.shallowCopyOf(LocalsHandler<T> other)
+      : locals = new VariableScope<T>.shallowCopyOf(other.locals),
+        fieldScope = new FieldInitializationScope<T>.from(other.fieldScope),
+        captured = other.captured,
+        capturedAndBoxed = other.capturedAndBoxed,
+        tryBlock = other.tryBlock,
+        types = other.types,
+        inferrer = other.inferrer,
+        compiler = other.compiler;
+
   T use(Local local) {
     if (capturedAndBoxed.containsKey(local)) {
       return inferrer.typeOfElement(capturedAndBoxed[local]);
@@ -906,11 +923,24 @@ abstract class InferrerVisitor
       LocalsHandler<T> saved = locals;
       locals = new LocalsHandler<T>.from(locals, node);
       updateIsChecks(isChecks, usePositive: true);
-      if (!oldAccumulateIsChecks) {
+      LocalsHandler<T> narrowed;
+      if (oldAccumulateIsChecks) {
+        narrowed = new LocalsHandler<T>.shallowCopyOf(locals);

[floitsch, 2015/01/21 14:34:02]

Would it be better to just copy out the locals?

It reaches more into the privates of the LocalsHandler, but on the other hand it
is not completely clear now, what "shallow" involves.

[herhut, 2015/01/21 14:45:19]

It only copies the top level, so I have renamed it to topLevelCopyOf to make
that clear. Even if I only do the locals, I need a way to copy one level deep
there, so I prefer to just do it with the whole thing. 

+      } else {
         accumulateIsChecks = false;
         isChecks = oldIsChecks;
       }
       visit(node.arguments.head);
+      if (oldAccumulateIsChecks) {
+        invalidatedInRightHandSide (Send test) {

[floitsch, 2015/01/21 14:34:02]

bool invalidate ...
new line before and after.

[herhut, 2015/01/21 14:45:19]

Done.

+          Element receiver = elements[test.receiver];
+          if (receiver is LocalElement) {
+            return narrowed.locals[receiver] != locals.locals[receiver];
+          }
+          return false;
+        }
+        isChecks.removeWhere(invalidatedInRightHandSide);
+      }
       saved.mergeDiamondFlow(locals, null);
       locals = saved;
       return types.boolType;