give `throwsToolExit` a more useful description (#136694)

Fixes https://github.com/flutter/flutter/issues/136698.

Alters how `throwToolExit` creates its matcher. This results is an improved description of the matcher.

The mismatch description isn't improved by this, but I writing an entirely custom matcher to fix this isn't ideal either. We can instead mitigate the issue by augmenting the `toString` implementation of `ToolExit` to include the exit code, if it is non-null.

With these changes, the first few lines of output from a test would look like this:

```
Expected: throws <Instance of 'ToolExit'> with `exitCode`: <42> and `message`: contains 'message'
  Actual: <Closure: () => Never>
   Which: threw ToolExit:<Exit code: 41232. Error: message>
```

packages/flutter_tools/lib/src/base/common.dart

@@ -21,5 +21,5 @@ class ToolExit implements Exception {
   final int? exitCode;
 
   @override
-  String toString() => 'Exception: $message'; // TODO(ianh): Really this should say "Error".
+  String toString() => 'Error: $message';
 }

packages/flutter_tools/test/general.shard/resident_runner_test.dart

@@ -1298,7 +1298,7 @@ flutter:
 
     await residentRunner.runSourceGenerators();
 
-    expect(testLogger.errorText, allOf(contains('Exception')));
+    expect(testLogger.errorText, contains('Error'));
     expect(testLogger.statusText, isEmpty);
   }));
 

packages/flutter_tools/test/integration.shard/debug_adapter/flutter_adapter_test.dart

@@ -184,7 +184,7 @@ void main() {
 
       final String output = _uniqueOutputLines(outputEvents);
       expect(output, contains('this code does not compile'));
-      expect(output, contains('Exception: Failed to build'));
+      expect(output, contains('Error: Failed to build'));
       expect(output, contains('Exited (1)'));
     });
 

packages/flutter_tools/test/src/common.dart

@@ -93,19 +93,20 @@ Future<StringBuffer> capturedConsolePrint(Future<void> Function() body) async {
 final Matcher throwsAssertionError = throwsA(isA<AssertionError>());
 
 /// Matcher for functions that throw [ToolExit].
+///
+/// [message] is matched using the [contains] matcher.
 Matcher throwsToolExit({ int? exitCode, Pattern? message }) {
-  Matcher matcher = _isToolExit;
+  TypeMatcher<ToolExit> result = const TypeMatcher<ToolExit>();
+
   if (exitCode != null) {
-    matcher = allOf(matcher, (ToolExit e) => e.exitCode == exitCode);
+    result = result.having((ToolExit e) => e.exitCode, 'exitCode', equals(exitCode));
   }
   if (message != null) {
-    matcher = allOf(matcher, (ToolExit e) => e.message?.contains(message) ?? false);
+    result = result.having((ToolExit e) => e.message, 'message', contains(message));
   }
-  return throwsA(matcher);
-}
 
-/// Matcher for [ToolExit]s.
-final TypeMatcher<ToolExit> _isToolExit = isA<ToolExit>();
+  return throwsA(result);
+}
 
 /// Matcher for functions that throw [UsageException].
 Matcher throwsUsageException({Pattern? message }) {