Flow analysis: Get rid of initialize and _freshVariableInfo.

The FlowModel._freshVariableInfo field was an optimization with
dubious benefit (it avoided a single allocation when seeing a variable
for the first time).  And it is getting in the way of changes I have
planned for dart-lang/language#1274 (which
will require each fresh variable to be allocated different
information).

Removing FlowModel._freshVariableInfo means that now we have a call to
VariableModel.fresh() whenever we see a variable for the first time,
so we can take advantage of this to elimiate the
VariableModel.initialize() method, and simply allocate the
VariableModel in the initialized state when it's appropriate to do so.

Change-Id: I4b6f7d004fff72eaa4025fdf79e9ca29a81d7f96
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/174566
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Reviewed-by: Johnni Winther <johnniwinther@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>

Author: stereotype441

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

@@ -1286,9 +1286,6 @@ class FlowModel<Variable, Type> {
   /// variable that is no longer in scope.
   final Map<Variable, VariableModel<Variable, Type> /*!*/ > variableInfo;
 
-  /// Variable model for variables that have never been seen before.
-  final VariableModel<Variable, Type> _freshVariableInfo;
-
   /// The empty map, used to [join] variables.
   final Map<Variable, VariableModel<Variable, Type>> _emptyVariableMap = {};
 
@@ -1302,8 +1299,7 @@ class FlowModel<Variable, Type> {
         );
 
   @visibleForTesting
-  FlowModel.withInfo(this.reachable, this.variableInfo)
-      : _freshVariableInfo = new VariableModel.fresh() {
+  FlowModel.withInfo(this.reachable, this.variableInfo) {
     assert(reachable != null);
     assert(() {
       for (VariableModel<Variable, Type> value in variableInfo.values) {
@@ -1376,17 +1372,15 @@ class FlowModel<Variable, Type> {
   /// A local variable is [initialized] if its declaration has an initializer.
   /// A function parameter is always initialized, so [initialized] is `true`.
   FlowModel<Variable, Type> declare(Variable variable, bool initialized) {
-    VariableModel<Variable, Type> newInfoForVar = _freshVariableInfo;
-    if (initialized) {
-      newInfoForVar = newInfoForVar.initialize();
-    }
+    VariableModel<Variable, Type> newInfoForVar =
+        new VariableModel.fresh(assigned: initialized);
 
     return _updateVariableInfo(variable, newInfoForVar);
   }
 
   /// Gets the info for the given [variable], creating it if it doesn't exist.
   VariableModel<Variable, Type> infoFor(Variable variable) =>
-      variableInfo[variable] ?? _freshVariableInfo;
+      variableInfo[variable] ?? new VariableModel.fresh();
 
   /// Builds a [FlowModel] based on `this`, but extending the `tested` set to
   /// include types from [other].  This is used at the bottom of certain kinds
@@ -2060,11 +2054,10 @@ class VariableModel<Variable, Type> {
 
   /// Creates a [VariableModel] representing a variable that's never been seen
   /// before.
-  VariableModel.fresh()
+  VariableModel.fresh({this.assigned = false})
       : promotedTypes = null,
         tested = const [],
-        assigned = false,
-        unassigned = true,
+        unassigned = !assigned,
         writeCaptured = false;
 
   /// Returns a new [VariableModel] in which any promotions present have been
@@ -2077,16 +2070,6 @@ class VariableModel<Variable, Type> {
         null, tested, assigned, false, writeCaptured);
   }
 
-  /// Returns a new [VariableModel] reflecting the fact that the variable was
-  /// just initialized.
-  VariableModel<Variable, Type> initialize() {
-    if (promotedTypes == null && tested.isEmpty && assigned && !unassigned) {
-      return this;
-    }
-    return new VariableModel<Variable, Type>(
-        null, const [], true, false, writeCaptured);
-  }
-
   /// Returns an updated model reflect a control path that is known to have
   /// previously passed through some [other] state.  See [FlowModel.restrict]
   /// for details.