[flow analysis] clean up public methods that call other public methods.

In a future CL, I plan to introduce some state tracking logic at the
top of each public method in `FlowAnalysisImpl` to verify that the
caller invokes flow analysis API methods in an order that respects
some important invariants. When I do that, it will be confusing if
there are some public methods in `FlowAnalysisImpl` that call other
public methods.

So, to avoid confusion in that future CL, this CL refactors the public
methods `lateInitializer_end` and `functionExpression_end` (which are
functionally equivalent) so that they call a common private method,
rather than one calling the other. Similarly, it refactors the public
methods `parenthesizedExpression` and `forwardExpression` (which are
functionally equivalent) so that they call a common private method,
rather than one calling the other.

Change-Id: I7b6d23f2db409c652575beb73bba2ea11b0fa90b
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/303200
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>

Author: stereotype441

pkg/_fe_analyzer_shared/lib/src/flow_analysis/flow_analysis.dart

@@ -4020,12 +4020,7 @@ class _FlowAnalysisImpl<Node extends Object, Statement extends Node,
 
   @override
   void forwardExpression(Expression newExpression, Expression oldExpression) {
-    if (identical(_expressionWithInfo, oldExpression)) {
-      _expressionWithInfo = newExpression;
-    }
-    if (identical(_expressionWithReference, oldExpression)) {
-      _expressionWithReference = newExpression;
-    }
+    _forwardExpression(newExpression, oldExpression);
   }
 
   @override
@@ -4041,9 +4036,7 @@ class _FlowAnalysisImpl<Node extends Object, Statement extends Node,
 
   @override
   void functionExpression_end() {
-    _SimpleContext<Type> context =
-        _stack.removeLast() as _FunctionExpressionContext<Type>;
-    _current = context._previous;
+    _functionExpression_end();
   }
 
   @override
@@ -4264,7 +4257,7 @@ class _FlowAnalysisImpl<Node extends Object, Statement extends Node,
     // `late x = LAZY_MAGIC(() => expr);` (where `LAZY_MAGIC` creates a lazy
     // evaluation thunk that gets replaced by the result of `expr` once it is
     // evaluated).
-    functionExpression_end();
+    _functionExpression_end();
   }
 
   @override
@@ -4423,7 +4416,7 @@ class _FlowAnalysisImpl<Node extends Object, Statement extends Node,
   @override
   void parenthesizedExpression(
       Expression outerExpression, Expression innerExpression) {
-    forwardExpression(outerExpression, innerExpression);
+    _forwardExpression(outerExpression, innerExpression);
   }
 
   @override
@@ -4917,6 +4910,21 @@ class _FlowAnalysisImpl<Node extends Object, Statement extends Node,
   ExpressionInfo<Type> _expressionEnd(Expression? expression) =>
       _getExpressionInfo(expression) ?? new _TrivialExpressionInfo(_current);
 
+  void _forwardExpression(Expression newExpression, Expression oldExpression) {
+    if (identical(_expressionWithInfo, oldExpression)) {
+      _expressionWithInfo = newExpression;
+    }
+    if (identical(_expressionWithReference, oldExpression)) {
+      _expressionWithReference = newExpression;
+    }
+  }
+
+  void _functionExpression_end() {
+    _SimpleContext<Type> context =
+        _stack.removeLast() as _FunctionExpressionContext<Type>;
+    _current = context._previous;
+  }
+
   /// Gets the [ExpressionInfo] associated with the [expression] (which should
   /// be the last expression that was traversed).  If there is no
   /// [ExpressionInfo] associated with the [expression], then `null` is