[analyzer_utilities] Add String.isPascalCase extension.

This will be used in a follow-up CL to recognize changes to the format
of `pkg/front_end/messages.yaml` and
`pkg/_fe_analyzer_shared/messages.yaml`.

Change-Id: I6a6a6964274aa116d9f0051ce7c099a9b8d84277
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/466444
Commit-Queue: Paul Berry <paulberry@google.com>
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>

pkg/analyzer_utilities/lib/extensions/string.dart

@@ -5,6 +5,7 @@
 extension StringExtension on String {
   static final _toSnakeCaseRegExp = RegExp('_?[A-Z]');
   static final _startsWithLowerCaseRegExp = RegExp('^[a-z]');
+  static final _startsWithUpperCaseRegExp = RegExp('^[A-Z]');
 
   /// Returns `true` if the string is a `camelCase` string.
   bool get isCamelCase {
@@ -13,6 +14,13 @@ extension StringExtension on String {
     return true;
   }
 
+  /// Returns `true` if the string is a `PascalCase` string.
+  bool get isPascalCase {
+    if (contains('_')) return false;
+    if (_startsWithUpperCaseRegExp.matchAsPrefix(this) == null) return false;
+    return true;
+  }
+
   /// Converts `SCREAMING_SNAKE_CASE` or `snake_case` to `camelCase`.
   String toCamelCase() {
     var parts = toLowerCase().split('_');

pkg/analyzer_utilities/test/extensions/string_test.dart

@@ -22,11 +22,26 @@ class StringExtensionTest {
     expect('UPPER_CASE_WITH_UNDERSCORES'.isCamelCase, false);
     expect('lower_case_with_underscores'.isCamelCase, false);
     expect('camelCase'.isCamelCase, true);
+    expect('PascalCase'.isCamelCase, false);
     expect('alllowercase'.isCamelCase, true);
+    expect('ALLUPPERCASE'.isCamelCase, false);
     expect('foo123Bar'.isCamelCase, true);
+    expect('Foo123Bar'.isCamelCase, false);
     expect('123'.isCamelCase, false);
   }
 
+  void test_isPascalCase() {
+    expect('UPPER_CASE_WITH_UNDERSCORES'.isPascalCase, false);
+    expect('lower_case_with_underscores'.isPascalCase, false);
+    expect('camelCase'.isPascalCase, false);
+    expect('PascalCase'.isPascalCase, true);
+    expect('alllowercase'.isPascalCase, false);
+    expect('ALLUPPERCASE'.isPascalCase, true);
+    expect('foo123Bar'.isPascalCase, false);
+    expect('Foo123Bar'.isPascalCase, true);
+    expect('123'.isPascalCase, false);
+  }
+
   void test_toCamelCase() {
     expect('CAMEL_CASE'.toCamelCase(), 'camelCase');
     expect('alreadyCamel_case'.toCamelCase(), 'alreadycamelCase');