[dart2js] fix crash in program emitter

The emitter creates classes in bulk, and then later connects them together
except for adding stubs for JS-interop is checks. We assumed the class for
JavaScriptObject was previously created and stored stubs eagerly while creating
classes.

We believe this caused a crash with flutter because the class was not yet
defined in that case.

One theory why this wasn't hit as much externally is that we sort classes by
location and process dart:* classes first. Flutter is the first use case where
JS-interop classes can be defined within SDK libraries.

Fixes #42612
Fixes #25517

Change-Id: Icad0a9a16ec0d05481ed60f581a23c9eeb1ed5d3
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/153943
Commit-Queue: Sigmund Cherem <sigmund@google.com>
Reviewed-by: Stephen Adams <sra@google.com>

Author: sigmundch

pkg/compiler/lib/src/js_emitter/program_builder/program_builder.dart

@@ -163,6 +163,8 @@ class ProgramBuilder {
 
   Set<Class> _unneededNativeClasses;
 
+  List<StubMethod> _jsInteropIsChecks = [];
+
   /// Classes that have been allocated during a profile run.
   ///
   /// These classes should not be soft-deferred.
@@ -492,6 +494,8 @@ class ProgramBuilder {
     // a method in the case where there exist multiple JavaScript classes
     // that conflict on whether the member is a getter or a method.
     Class interceptorClass = _classes[_commonElements.jsJavaScriptObjectClass];
+
+    interceptorClass?.isChecks?.addAll(_jsInteropIsChecks);
     Set<String> stubNames = {};
     librariesMap
         .forEach((LibraryEntity library, List<ClassEntity> classElements, _) {
@@ -750,9 +754,7 @@ class ProgramBuilder {
       // Currently we generate duplicates if a class is implemented by multiple
       // js-interop classes.
       typeTests.forEachProperty(_sorter, (js.Name name, js.Node code) {
-        _classes[_commonElements.jsJavaScriptObjectClass]
-            .isChecks
-            .add(_buildStubMethod(name, code));
+        _jsInteropIsChecks.add(_buildStubMethod(name, code));
       });
     } else {
       for (Field field in instanceFields) {