Push logic for constraining argument types into argument visiting loop.

In order to implement dart-lang/language#731
(improved inference for fold etc.), we'll need to gradually accumulate
type constraints as we evaluate arguments, and periodically re-do
inference; we can't just accumulate all the formal types and actual
types and run them through an inference process at the end.

This change moves us a step toward that eventuality, by accumulating
type constraints after type inference visits each argument, rather
than all at once after all arguments have been visited.  The total
amount of work done is unchanged.

Change-Id: I91ed0529cd3142afe4153cac8c25bce3c20f137d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/241800
Reviewed-by: Chloe Stefantsova <[email protected]>
Commit-Queue: Paul Berry <[email protected]>

Author: stereotype441

pkg/front_end/lib/src/fasta/type_inference/type_inferrer.dart

@@ -2287,8 +2287,7 @@ class TypeInferrerImpl implements TypeInferrer {
         explicitTypeArguments == null &&
         calleeTypeParameters.isNotEmpty;
     bool typeChecksNeeded = !isTopLevel;
-    bool useFormalAndActualTypes = inferenceNeeded ||
-        typeChecksNeeded ||
+    bool useFormalAndActualTypes = typeChecksNeeded ||
         isSpecialCasedBinaryOperator ||
         isSpecialCasedTernaryOperator;
 
@@ -2472,6 +2471,7 @@ class TypeInferrerImpl implements TypeInferrer {
         NamedExpression namedArgument = arguments.named[index];
         namedArgument.value = expression..parent = namedArgument;
       }
+      gatherer?.tryConstrainLower(formalType, inferredType);
       if (useFormalAndActualTypes) {
         formalTypes!.add(formalType);
         actualTypes!.add(inferredType);
@@ -2574,8 +2574,7 @@ class TypeInferrerImpl implements TypeInferrer {
     }
 
     if (inferenceNeeded) {
-      gatherer!.constrainArguments(formalTypes!, actualTypes!);
-      typeSchemaEnvironment.upwardsInfer(gatherer, calleeTypeParameters,
+      typeSchemaEnvironment.upwardsInfer(gatherer!, calleeTypeParameters,
           inferredTypes!, libraryBuilder.library);
       assert(inferredTypes.every((type) => isKnown(type)),
           "Unknown type(s) in inferred types: $inferredTypes.");