Fix inference bugs found by sra@ by changing how we merge if/then/else locals.

sdk/lib/_internal/compiler/implementation/types/inferrer_visitor.dart

Index: sdk/lib/_internal/compiler/implementation/types/inferrer_visitor.dart
===================================================================
--- sdk/lib/_internal/compiler/implementation/types/inferrer_visitor.dart	(revision 26109)
+++ sdk/lib/_internal/compiler/implementation/types/inferrer_visitor.dart	(working copy)
@@ -243,8 +243,9 @@
     forEachLocalUntilNode(null, f);
   }
 
-  void remove(Element element) {
-    variables.remove(element);
+  bool updates(Element element) {
+    if (variables == null) return false;
+    return variables.containsKey(element);
   }
 
   String toString() {
@@ -399,21 +400,43 @@
         && elseBranch != null
         && elseBranch.seenBreakOrContinue;
     if (aborts) return;
+
+    void mergeOneBranch(LocalsHandler<T> other) {
+      other.locals.forEachOwnLocal((Element local, T type) {
+        T myType = locals[local];
+        if (myType == null) return;
+        if (type == myType) return;
+        locals[local] = types.allocateDiamondPhi(myType, type);
+      });
+    }
+
     if (thenBranch.aborts) {
-      thenBranch = this;
       if (elseBranch == null) return;
+      mergeOneBranch(elseBranch);
     } else if (elseBranch == null || elseBranch.aborts) {
-      elseBranch = this;
+      mergeOneBranch(thenBranch);
+    } else {
+      void mergeLocal(Element local) {
+        T myType = locals[local];
+        if (myType == null) return;
+        T elseType = elseBranch.locals[local];
+        T thenType = thenBranch.locals[local];
+        if (thenType == elseType) {
+          locals[local] = thenType;
+        } else {
+          locals[local] = types.allocateDiamondPhi(thenType, elseType);
+        }
+      }
+
+      thenBranch.locals.forEachOwnLocal((Element local, _) {
+        mergeLocal(local);
+      });
+      elseBranch.locals.forEachOwnLocal((Element local, _) {
+        // Discard locals we already processed when iterating over
+        // [thenBranch]'s locals.
+        if (!thenBranch.locals.updates(local)) mergeLocal(local);
+      });
     }
-
-    thenBranch.locals.forEachOwnLocal((Element local, T type) {
-      T otherType = elseBranch.locals[local];
-      if (otherType == null) return;
-      T existing = locals[local];
-      if (type == existing && otherType == existing) return;
-      locals[local] = types.allocateDiamondPhi(type, otherType);
-    });
-
   }
 
   /**