Fix bug in our SSA optimizations: by giving an empty type to a box, we were assuming that anything …

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

Index: sdk/lib/_internal/compiler/implementation/ssa/value_range_analyzer.dart
===================================================================
--- sdk/lib/_internal/compiler/implementation/ssa/value_range_analyzer.dart	(revision 29654)
+++ sdk/lib/_internal/compiler/implementation/ssa/value_range_analyzer.dart	(working copy)
@@ -680,6 +680,8 @@
     if (!fieldGet.receiver.isIndexable(compiler)) {
       return visitInstruction(fieldGet);
     }
+    JavaScriptBackend backend = compiler.backend;
+    assert(fieldGet.element == backend.jsIndexableLength);
     LengthValue value = info.newLengthValue(fieldGet);
     // We know this range is above zero. To simplify the analysis, we
     // put the zero value as the lower bound of this range. This