[cfe] Implement a desugaring for simple if-case statements

Part of #49749

Change-Id: I38c486d4982c9dbd5515c36eb8c38cc76ff09800
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/265760
Reviewed-by: Johnni Winther <[email protected]>
Commit-Queue: Chloe Stefantsova <[email protected]>

Author: chloestefantsova

pkg/front_end/lib/src/fasta/kernel/body_builder.dart

@@ -2317,6 +2317,11 @@ class BodyBuilder extends StackListenerImpl
           libraryFeatures.patterns, case_.charOffset, case_.charCount);
       Matcher matcher = toMatcher(pop());
       Expression expression = popForValue();
+      for (VariableDeclaration variable in matcher.declaredVariables) {
+        // Skip synthetic variables.
+        if (variable.name == null) continue;
+        declareVariable(variable, scope);
+      }
       push(new Condition(expression, matcher, guard));
     } else {
       assert(checkState(token, [
@@ -8331,8 +8336,20 @@ class BodyBuilder extends StackListenerImpl
         libraryFeatures.patterns, variable.charOffset, variable.charCount);
     // ignore: unused_local_variable
     TypeBuilder? type = pop(NullValue.TypeBuilder) as TypeBuilder?;
-    // TODO(johnniwinther): Create a variable binder
-    push(new DummyBinder());
+    DartType? patternType = type?.build(libraryBuilder, TypeUse.variableType);
+    Binder binder;
+    if (variable.lexeme == "_") {
+      binder = new WildcardBinder(patternType, offset: variable.charOffset);
+    } else {
+      binder = new VariableBinder(
+          patternType,
+          variable.lexeme,
+          forest.createVariableDeclaration(variable.charOffset, variable.lexeme,
+              type: patternType,
+              isFinal: Modifier.validateVarFinalOrConst(keyword?.lexeme) ==
+                  finalMask));
+    }
+    push(binder);
   }
 
   @override

pkg/front_end/lib/src/fasta/kernel/forest.dart

@@ -664,7 +664,8 @@ class Forest {
         isInitializingFormal: isInitializingFormal,
         isCovariantByDeclaration: isCovariantByDeclaration,
         isLocalFunction: isLocalFunction,
-        hasDeclaredInitializer: initializer != null);
+        hasDeclaredInitializer: initializer != null)
+      ..fileOffset = fileOffset;
   }
 
   VariableDeclarationImpl createVariableDeclarationForValue(
@@ -805,6 +806,20 @@ class Forest {
       ..fileOffset = fileOffset;
   }
 
+  VariableGet createVariableGet(int fileOffset, VariableDeclaration variable) {
+    // ignore: unnecessary_null_comparison
+    assert(fileOffset != null);
+    return new VariableGetImpl(variable, forNullGuardedAccess: false)
+      ..fileOffset = fileOffset;
+  }
+
+  VariableSet createVariableSet(
+      int fileOffset, VariableDeclaration variable, Expression value) {
+    // ignore: unnecessary_null_comparison
+    assert(fileOffset != null);
+    return new VariableSet(variable, value)..fileOffset = fileOffset;
+  }
+
   EqualsExpression createEquals(
       int fileOffset, Expression left, Expression right,
       {required bool isNot}) {