[sound flow analysis] Implement behaviors for map patterns.

This change updates the flow analysis logic for map patterns, so that
when the language feature `sound-flow-analysis` is enabled, an empty
map pattern is considered to match a non-nullable map.

There is no behavioral change if the feature `sound-flow-analysis` is
disabled.

Bug: #60438
Change-Id: I3f5a79c00cfe91d37528a3790b5cedb9a8f010fc
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/421584
Reviewed-by: Konstantin Shcheglov <[email protected]>
Commit-Queue: Paul Berry <[email protected]>

Author: stereotype441

pkg/_fe_analyzer_shared/lib/src/type_inference/type_analyzer.dart

@@ -1090,10 +1090,16 @@ mixin TypeAnalyzer<
       keyType: keyType,
       valueType: valueType,
     );
+    bool matchMayFailEvenIfCorrectType = true;
+    if (typeAnalyzerOptions.soundFlowAnalysisEnabled && elements.isEmpty) {
+      // With sound null safety, an empty map pattern can only fail to match if
+      // the types don't match.
+      matchMayFailEvenIfCorrectType = false;
+    }
     flow.promoteForPattern(
         matchedType: matchedValueType,
         knownType: requiredType,
-        matchMayFailEvenIfCorrectType: true);
+        matchMayFailEvenIfCorrectType: matchMayFailEvenIfCorrectType);
     // Stack: ()
 
     Map<int, Error>? restPatternErrors;

pkg/_fe_analyzer_shared/test/flow_analysis/flow_analysis_test.dart

@@ -11051,6 +11051,35 @@ main() {
         });
       });
     });
+
+    group('Map pattern', () {
+      test('When enabled, guaranteed to match non-nullable map', () {
+        h.run([
+          ifCase(
+              expr('Map<int, int>'), mapPattern([])..errorId = 'mapPattern', [
+            checkReachable(true),
+          ], [
+            checkReachable(false),
+          ])
+        ], expectedErrors: {
+          'emptyMapPattern(pattern: mapPattern)'
+        });
+      });
+
+      test('When disabled, not guaranteed to match non-nullable map', () {
+        h.disableSoundFlowAnalysis();
+        h.run([
+          ifCase(
+              expr('Map<int, int>'), mapPattern([])..errorId = 'mapPattern', [
+            checkReachable(true),
+          ], [
+            checkReachable(true),
+          ])
+        ], expectedErrors: {
+          'emptyMapPattern(pattern: mapPattern)'
+        });
+      });
+    });
   });
 }