[analyzer_utilities] Add String.toSnakeCase extension method.

This extension method complements the existing `toCamelCase` and
`toPascalCase` extension methods, allowing a string to be converted
from `camelCase` or `PascalCase` to `snake_case`.

This extension method will be useful in the analyzer diagnostic
message generation logic, which still uses a mix of case styles.

Note that there's no need for a separate method to convert to
`UPPER_SNAKE_CASE`; that can be easily done by calling `toUpperCase`
after `toSnakeCase`.

Change-Id: I6a6a69647f4ee176ecdbaefc3de0906a88c69079
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/460206
Reviewed-by: Johnni Winther <johnniwinther@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>

pkg/analyzer_utilities/lib/extensions/string.dart

@@ -3,6 +3,8 @@
 // BSD-style license that can be found in the LICENSE file.
 
 extension StringExtension on String {
+  static final _toSnakeCaseRegExp = RegExp('_?[A-Z]');
+
   /// Converts `SCREAMING_SNAKE_CASE` or `snake_case` to `camelCase`.
   String toCamelCase() {
     var parts = toLowerCase().split('_');
@@ -51,4 +53,26 @@ extension StringExtension on String {
     }
     return buffer.toString();
   }
+
+  /// Converts `camelCase` or `PascalCase` to `snake_case`
+  String toSnakeCase() {
+    var parts = <String>[];
+    var i = 0;
+    var wordStarts = _toSnakeCaseRegExp.allMatches(this);
+    for (var RegExpMatch(:start) in wordStarts) {
+      if (i < start) {
+        parts.add(substring(i, start).toLowerCase());
+        i = start;
+      }
+      if (this[i] == '_' && parts.isNotEmpty) {
+        // Avoid doubling up the `_`. This handles strings that are already in
+        // snake case like `foo_Bar` (which translates to `foo_bar`).
+        i++;
+      }
+    }
+    if (i < length) {
+      parts.add(substring(i).toLowerCase());
+    }
+    return parts.join('_');
+  }
 }

pkg/analyzer_utilities/test/extensions/string_test.dart

@@ -41,4 +41,20 @@ class StringExtensionTest {
     expect('FOO__BAR'.toPascalCase(), 'FooBar');
     expect('FOO_BAR_'.toPascalCase(), 'FooBar');
   }
+
+  void test_toSnakeCase() {
+    expect('camelCase'.toSnakeCase(), 'camel_case');
+    expect('PascalCase'.toSnakeCase(), 'pascal_case');
+    expect('already_snake_case'.toSnakeCase(), 'already_snake_case');
+    expect(
+      'mixedCamel_AndPascal_and_snake'.toSnakeCase(),
+      'mixed_camel_and_pascal_and_snake',
+    );
+    expect('with123Numbers'.toSnakeCase(), 'with123_numbers');
+    expect(''.toSnakeCase(), '');
+    expect(
+      'CONSECUTIVE_UPCASE'.toSnakeCase(),
+      'c_o_n_s_e_c_u_t_i_v_e_u_p_c_a_s_e',
+    );
+  }
 }