Suggest a solution when Flutter isn't available (#1720)

Closes #1719

Author: nex3

lib/src/source/sdk.dart

@@ -95,7 +95,9 @@ class BoundSdkSource extends BoundSource {
   /// contain the package.
   String _verifiedPackagePath(String name) {
     if (!flutter.isAvailable) {
-      throw new PackageNotFoundException('The Flutter SDK is not available.');
+      throw new PackageNotFoundException("The Flutter SDK is not available.\n"
+          "Flutter users should run `flutter packages get` instead of `pub "
+          "get`.");
     }
 
     var path = flutter.packagePath(name);

test/sdk_test.dart

@@ -96,6 +96,8 @@ main() {
         }).create();
         await pubCommand(command,
             error: 'The Flutter SDK is not available.\n'
+                'Flutter users should run `flutter packages get` instead of '
+                '`pub get`.\n'
                 'Depended on by:\n'
                 '- myapp',
             exitCode: exit_codes.UNAVAILABLE);