Validate that all instructions dominate their inputs. And fix a bug where that did not happen.

lib/compiler/implementation/ssa/nodes.dart

Index: lib/compiler/implementation/ssa/nodes.dart
===================================================================
--- lib/compiler/implementation/ssa/nodes.dart	(revision 8031)
+++ lib/compiler/implementation/ssa/nodes.dart	(working copy)
@@ -661,6 +661,14 @@
     }
   }
 
+  void forEachInstruction(void f(HInstruction instruction)) {
+    HInstruction current = first;
+    while (current !== null) {
+      f(current);
+      current = current.next;
+    }
+  }
+
   bool isValid() {
     assert(isClosed());
     HValidator validator = new HValidator();
@@ -1664,6 +1672,7 @@
 
 class HTry extends HControlFlow {
   HParameterValue exception;
+  HBasicBlock catchBlock;
   HBasicBlock finallyBlock;
   HTry() : super(const <HInstruction>[]);
   toString() => 'try';