[dart2js] Remove trailing whitespace and newline from error message.

Before:

```
dart compile js --wrong-flag
Compile Dart to JavaScript.

Usage: dart compile js [arguments] <dart entry point>
  -h, --help      Print this usage information.
  -h -v           Show detailed information about all options.
  -o, --output    Write the output to <file name>.
  -O<0,1,2,3,4>   Set the compiler optimization level (defaults to -O1).
     -O0          No optimizations (only meant for debugging the compiler).
     -O1          Default (includes whole program analyses and inlining).
     -O2          Safe production-oriented optimizations (like minification).
     -O3          Potentially unsafe optimizations (see -h -v for details).
     -O4          More agressive unsafe optimizations (see -h -v for details).
  <-- TWO SPACES HERE and a newline

Error: Unknown option '--wrong-flag'.
```

After this change:

```
dart compile js --wrong-flag
Compile Dart to JavaScript.

Usage: dart compile js [arguments] <dart entry point>
  -h, --help      Print this usage information.
  -h -v           Show detailed information about all options.
  -o, --output    Write the output to <file name>.
  -O<0,1,2,3,4>   Set the compiler optimization level (defaults to -O1).
     -O0          No optimizations (only meant for debugging the compiler).
     -O1          Default (includes whole program analyses and inlining).
     -O2          Safe production-oriented optimizations (like minification).
     -O3          Potentially unsafe optimizations (see -h -v for details).
     -O4          More agressive unsafe optimizations (see -h -v for details).

Error: Unknown option '--wrong-flag'.
```
[email protected]
Change-Id: Ie0541d830b53e3e81b4b02a20988cbd41db64325
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/339300
Auto-Submit: Morgan :) <[email protected]>
Reviewed-by: Stephen Adams <[email protected]>
Commit-Queue: Morgan :) <[email protected]>

Author: davidmorgan

pkg/compiler/lib/src/dart2js.dart

@@ -14,9 +14,9 @@ import 'package:front_end/src/api_unstable/dart2js.dart' as fe;
 import '../compiler_api.dart' as api;
 import 'commandline_options.dart';
 import 'common/ram_usage.dart';
+import 'compiler.dart' as defaultCompiler show Compiler;
 import 'io/mapped_file.dart';
 import 'options.dart' show CompilerOptions, Dart2JSStage, FeatureOptions;
-import 'compiler.dart' as defaultCompiler show Compiler;
 import 'source_file_provider.dart';
 import 'util/command_line.dart';
 import 'util/util.dart' show stackTraceFilePrefix;
@@ -982,7 +982,8 @@ Usage: dart compile js [arguments] <dart entry point>
      -O2          Safe production-oriented optimizations (like minification).
      -O3          Potentially unsafe optimizations (see -h -v for details).
      -O4          More agressive unsafe optimizations (see -h -v for details).
-  ''');
+'''
+      .trim());
 }
 
 void verboseHelp() {