SDK master=>main default branch migration (#1408)

* master=>main

* Update location.dart

* Update debugger.dart

Author: pq

dwds/README.md

@@ -9,6 +9,6 @@ with the native Dart VM to also work with Dart Web applications compiled with
 
 At a basic level, DWDS proxies between:
 * Developer tools that are written against the
-  [Dart VM Service Protocol](https://github.com/dart-lang/sdk/blob/master/runtime/vm/service/service.md)
+  [Dart VM Service Protocol](https://github.com/dart-lang/sdk/blob/main/runtime/vm/service/service.md)
 * Execution environments that expose the
   [Chrome DevTools Protocol](https://chromedevtools.github.io/devtools-protocol)

dwds/lib/src/debugging/debugger.dart

@@ -101,7 +101,7 @@ class Debugger extends Domain {
   /// Resumes the debugger.
   ///
   /// Step parameter options:
-  /// https://github.com/dart-lang/sdk/blob/master/runtime/vm/service/service.md#resume
+  /// https://github.com/dart-lang/sdk/blob/main/runtime/vm/service/service.md#resume
   ///
   /// If the step parameter is not provided, the program will resume regular
   /// execution.

dwds/lib/src/debugging/location.dart

@@ -112,7 +112,7 @@ class Locations {
 
   /// `tokenPosTable` for Dart server path, as defined in the
   /// Dart VM Service Protocol:
-  /// https://github.com/dart-lang/sdk/blob/master/runtime/vm/service/service.md#script
+  /// https://github.com/dart-lang/sdk/blob/main/runtime/vm/service/service.md#script
   final Map<String, List<List<int>>> _sourceToTokenPosTable = {};
 
   /// The set of all known [Location]s for a module.
@@ -170,7 +170,7 @@ class Locations {
 
   /// Returns the tokenPosTable for the provided Dart script path as defined
   /// in:
-  /// https://github.com/dart-lang/sdk/blob/master/runtime/vm/service/service.md#script
+  /// https://github.com/dart-lang/sdk/blob/main/runtime/vm/service/service.md#script
   Future<List<List<int>>> tokenPosTableFor(String serverPath) async {
     var tokenPosTable = _sourceToTokenPosTable[serverPath];
     if (tokenPosTable != null) return tokenPosTable;