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 3ffbda294e5869cd926308f9e059a3381ae26bd4..114149595130c37837c0dfb79eafda1e20d99379 100644
--- a/pkg/dev_compiler/lib/js/legacy/dart_library.js
+++ b/pkg/dev_compiler/lib/js/legacy/dart_library.js
@@ -76,7 +76,11 @@ 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;
     }
@@ -166,6 +170,14 @@ dart_library =
   }
   dart_library.library = library;
 
+<<<<<<< HEAD
+  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();
+=======
   // 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 = [];
@@ -187,6 +199,7 @@ dart_library =
     let result = lib.load();
     _stack.pop();
     return result;
+>>>>>>> origin/master
   }
   dart_library.import = import_;