[cfe] Implement new async return rules

Closes #41800
Closes #41900
Closes #42134
Closes #42282
Closes #42236
Closes #42169

Change-Id: Ia994bc07fba4e2342fcb59d44fc77608198a328b
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/152150
Commit-Queue: Johnni Winther <johnniwinther@google.com>
Reviewed-by: Dmitry Stefantsov <dmitryas@google.com>

Author: johnniwinther

pkg/_fe_analyzer_shared/lib/src/messages/codes_generated.dart

@@ -7541,6 +7541,26 @@ const MessageCode messageReturnWithoutExpression = const MessageCode(
     severity: Severity.warning,
     message: r"""Must explicitly return a value from a non-void function.""");
 
+// DO NOT EDIT. THIS FILE IS GENERATED. SEE TOP OF FILE.
+const Code<Null> codeReturnWithoutExpressionAsync =
+    messageReturnWithoutExpressionAsync;
+
+// DO NOT EDIT. THIS FILE IS GENERATED. SEE TOP OF FILE.
+const MessageCode messageReturnWithoutExpressionAsync = const MessageCode(
+    "ReturnWithoutExpressionAsync",
+    message:
+        r"""A value must be explicitly returned from a non-void async function.""");
+
+// DO NOT EDIT. THIS FILE IS GENERATED. SEE TOP OF FILE.
+const Code<Null> codeReturnWithoutExpressionSync =
+    messageReturnWithoutExpressionSync;
+
+// DO NOT EDIT. THIS FILE IS GENERATED. SEE TOP OF FILE.
+const MessageCode messageReturnWithoutExpressionSync = const MessageCode(
+    "ReturnWithoutExpressionSync",
+    message:
+        r"""A value must be explicitly returned from a non-void function.""");
+
 // DO NOT EDIT. THIS FILE IS GENERATED. SEE TOP OF FILE.
 const Template<Message Function(Uri uri_)> templateSdkRootNotFound =
     const Template<Message Function(Uri uri_)>(

pkg/compiler/lib/src/ir/static_type_base.dart

@@ -100,7 +100,7 @@ abstract class StaticTypeBase extends ir.Visitor<ir.DartType> {
 
   @override
   ir.DartType visitAwaitExpression(ir.AwaitExpression node) {
-    return typeEnvironment.unfutureType(visitNode(node.operand));
+    return typeEnvironment.flatten(visitNode(node.operand));
   }
 
   @override

pkg/dev_compiler/lib/src/kernel/compiler.dart

@@ -3125,7 +3125,7 @@ class ProgramCompiler extends ComputeOnceConstantVisitor<js_ast.Expression>
     var gen = emitGeneratorFn((_) => []);
     // Return type of an async body is `Future<flatten(T)>`, where T is the
     // declared return type.
-    var returnType = _types.unfutureType(function
+    var returnType = _types.flatten(function
         .computeThisFunctionType(_currentLibrary.nonNullable)
         .returnType);
     return js.call('#.async(#, #)',

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

@@ -271,7 +271,7 @@ class InferenceVisitor
         node.operand, typeContext, true,
         isVoidAllowed: !inferrer.isNonNullableByDefault);
     DartType inferredType =
-        inferrer.typeSchemaEnvironment.unfutureType(operandResult.inferredType);
+        inferrer.typeSchemaEnvironment.flatten(operandResult.inferredType);
     node.operand = operandResult.expression..parent = node;
     return new ExpressionInferenceResult(inferredType, node);
   }
@@ -2161,7 +2161,7 @@ class InferenceVisitor
     bool typeContextIsMap = node.keyType is! ImplicitTypeArgument;
     bool typeContextIsIterable = false;
     DartType unfuturedTypeContext =
-        inferrer.typeSchemaEnvironment.unfutureType(typeContext);
+        inferrer.typeSchemaEnvironment.flatten(typeContext);
     if (!inferrer.isTopLevel && inferenceNeeded) {
       // Ambiguous set/map literal
       if (unfuturedTypeContext is InterfaceType) {