Merge CL for js-interop crash fix.

pkg/compiler/lib/src/js_backend/js_interop_analysis.dart

Index: pkg/compiler/lib/src/js_backend/js_interop_analysis.dart
diff --git a/pkg/compiler/lib/src/js_backend/js_interop_analysis.dart b/pkg/compiler/lib/src/js_backend/js_interop_analysis.dart
index e6cce5b2340443fc5c9c5efc9c5c52b06894845b..4199c691e882808d5bfe64a15df9a2038c5bf4eb 100644
--- a/pkg/compiler/lib/src/js_backend/js_interop_analysis.dart
+++ b/pkg/compiler/lib/src/js_backend/js_interop_analysis.dart
@@ -116,6 +116,10 @@ class JsInteropAnalysis {
 
       ClassElement classElement = element;
 
+      // Skip classes that are completely unreachable. This should only happen
+      // when all of jsinterop types are unreachable from main.
+      if (!backend.compiler.world.isImplemented(classElement)) return;
+
       if (!classElement
           .implementsInterface(backend.jsJavaScriptObjectClass)) {
         backend.reporter.reportErrorMessage(classElement,