Fix crash in compiler when we are generating a bailout method where a loop contains continue/break/…

sdk/lib/_internal/compiler/implementation/ssa/nodes.dart

Index: sdk/lib/_internal/compiler/implementation/ssa/nodes.dart
===================================================================
--- sdk/lib/_internal/compiler/implementation/ssa/nodes.dart	(revision 17824)
+++ sdk/lib/_internal/compiler/implementation/ssa/nodes.dart	(working copy)
@@ -2589,7 +2589,10 @@
                         this.target,
                         this.labels,
                         this.sourcePosition,
-                        this.endSourcePosition);
+                        this.endSourcePosition) {
+    assert(
+        (kind == DO_WHILE_LOOP ? body.start : condition.start).isLoopHeader());
+  }
 
   HBasicBlock get start {
     if (initializer != null) return initializer.start;