Remove warnings when building the dart2js sdk.

tools/patch_sdk.dart

 #!/usr/bin/env dart
 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 /// Command line tool to merge the SDK libraries and our patch files.
 /// This is currently designed as an offline tool, but we could automate it.
 
 import 'dart:io';
 import 'dart:isolate' show RawReceivePort;
@@ skipping 579 matching lines @@
   }
 
   void _maybePatch(AstNode node) {
     if (node is FieldDeclaration) return;
 
     var externalKeyword = (node as dynamic).externalKeyword;
 
     var name = _qualifiedName(node);
     var patchNode = patch.patches[name];
     if (patchNode == null) {
-      if (externalKeyword != null) {
+      if (externalKeyword != null && _shouldHaveImplementation(name)) {
         print('warning: patch not found for $name: $node');
+        exitCode = 1;
       }
       return;
     }
     patch.applied.add(name);
 
     Annotation patchMeta = patchNode.metadata.lastWhere(_isPatchAnnotation);
     int start = patchMeta.endToken.next.offset;
     var code = patch.contents.substring(start, patchNode.end);
 
     // For some node like static fields, the node's offset doesn't include
     // the external keyword. Also starting from the keyword lets us preserve
     // documentation comments.
     edits.replace(externalKeyword?.offset ?? node.offset, node.end, code);
   }
 }
 
+/// Whether a member should have an implementation after patching the SDK.
+///
+/// True for most members except for the *.fromEnvironment constructors under
+/// the dart2js target.
+bool _shouldHaveImplementation(String qualifiedName) {
+  if (!forDart2js) return true;
+  // Note: dart2js implements int.fromEnvironment, bool.fromEnvironment
+  // and String.fromEnvironment directly and expects the SDK code to
+  // have an external declaration.
+  var isFromEnvironment = const [
+    'bool.fromEnvironment',
+    'int.fromEnvironment',
+    'String.fromEnvironment'
+  ].contains(qualifiedName);
+  return !isFromEnvironment;
+}
+
 class PatchFinder extends GeneralizingAstVisitor {
   final String contents;
   final CompilationUnit unit;
 
   final Map patches = <String, Declaration>{};
   final Map mergeMembers = <String, List<ClassMember>>{};
   final List mergeDeclarations = <CompilationUnitMember>[];
   final Set<String> applied = new Set<String>();
 
   PatchFinder.parseAndVisit(String name, String contents)
@@ skipping 153 matching lines @@
     if (diff != 0) return diff;
     return end - other.end;
   }
 }
 
 List<SdkLibrary> _getSdkLibraries(String contents) {
   var libraryBuilder = new SdkLibrariesReader_LibraryBuilder(forDart2js);
   parseCompilationUnit(contents).accept(libraryBuilder);
   return libraryBuilder.librariesMap.sdkLibraries;
 }