do not suggest elements from internal sdk libs

pkg/analysis_server/test/mock_sdk.dart

Index: pkg/analysis_server/test/mock_sdk.dart
diff --git a/pkg/analysis_server/test/mock_sdk.dart b/pkg/analysis_server/test/mock_sdk.dart
index abc6a3c05bdada3f50499c4e367f6ecde7383906..203cf8f254e5f3fb6bb56557ffbc5974e3acd4a4 100644
--- a/pkg/analysis_server/test/mock_sdk.dart
+++ b/pkg/analysis_server/test/mock_sdk.dart
@@ -16,6 +16,7 @@ class MockSdk implements DartSdk {
 library dart.core;
 
 import 'dart:async';
+import 'dart:_internal';
 
 class Object {
   bool operator ==(other) => identical(this, other);
@@ -144,6 +145,7 @@ const double LN10 =  2.302585092994046;
 num min(num a, num b) => 0;
 num max(num a, num b) => 0;
 external double cos(num x);
+external num pow(num x, num exponent);
 external double sin(num x);
 external double sqrt(num x);
 class Random {
@@ -159,6 +161,12 @@ library dart.html;
 class HtmlElement {}
 ''');
 
+  static const _MockSdkLibrary LIB_INTERNAL = const _MockSdkLibrary(
+      'dart:_internal', '/lib/internal/internal.dart', '''
+library dart._internal;
+external void printToConsole(String line);
+''');
+
   static const List<SdkLibrary> LIBRARIES = const [
     LIB_CORE,
     LIB_ASYNC,
@@ -166,6 +174,7 @@ class HtmlElement {}
     LIB_CONVERT,
     LIB_MATH,
     LIB_HTML,
+    LIB_INTERNAL,
   ];
 
   final resource.MemoryResourceProvider provider =
@@ -264,7 +273,8 @@ class HtmlElement {}
       "dart:async": "/lib/async/async.dart",
       "dart:collection": "/lib/collection/collection.dart",
       "dart:convert": "/lib/convert/convert.dart",
-      "dart:math": "/lib/math/math.dart"
+      "dart:math": "/lib/math/math.dart",
+      "dart:_internal": "/lib/internal/internal.dart",
     };
 
     String path = uriToPath[dartUri];
@@ -300,7 +310,7 @@ class _MockSdkLibrary implements SdkLibrary {
   bool get isImplementation => throw unimplemented;
 
   @override
-  bool get isInternal => throw unimplemented;
+  bool get isInternal => shortName.startsWith('dart:_');
 
   @override
   bool get isShared => throw unimplemented;