fix #27259, implement covariance checking for strong mode and DDC

pkg/dev_compiler/lib/js/legacy/dart_library.js

Index: pkg/dev_compiler/lib/js/legacy/dart_library.js
diff --git a/pkg/dev_compiler/lib/js/legacy/dart_library.js b/pkg/dev_compiler/lib/js/legacy/dart_library.js
index 9c6a376eb12b3a15931ffc07a565e73660efa08a..89549ae240d6475f998a2f371ab4fc99a8d71436 100644
--- a/pkg/dev_compiler/lib/js/legacy/dart_library.js
+++ b/pkg/dev_compiler/lib/js/legacy/dart_library.js
@@ -67,7 +67,11 @@ dart_library =
     loadImports() {
       let results = [];
       for (let name of this._imports) {
-        results.push(import_(name));
+        let lib = libraries.get(name);
+        if (!lib) {
+          throwLibraryError('Library not available: ' + name);
+        }
+        results.push(lib.load());
       }
       return results;
     }
@@ -148,27 +152,12 @@ dart_library =
   }
   dart_library.library = library;
 
-  // Maintain a stack of active imports.  If a requested library/module is not
-  // available, print the stack to show where/how it was requested.
-  let _stack = [];
-  function import_(name) {
-    let lib = libraries.get(name);
-    if (!lib) {
-      let message = 'Module ' + name + ' not loaded in the browser.';
-      if (_stack != []) {
-        message += '\nDependency via:';
-        let indent = '';
-        for (let last = _stack.length - 1; last >= 0; last--) {
-          indent += ' ';
-          message += '\n' + indent + '- ' + _stack[last];
-        }
-      }
-      throwLibraryError(message);
-    }
-    _stack.push(name);
-    let result = lib.load();
-    _stack.pop();
-    return result;
+  function import_(libraryName) {
+    let loader = libraries.get(libraryName);
+    // TODO(vsm): A user might call this directly from JS (as we do in tests).
+    // We may want a different error type.
+    if (!loader) throwLibraryError('Library not found: ' + libraryName);
+    return loader.load();
   }
   dart_library.import = import_;