Use flow analysis to report potentially non-nullable local variables that are used before definitely assigned.

Not done yet:
1. Switching all unit tests to `performFlowAnalysis()`.
2. Updating the error code and renaming tests.
3. Writing more language_2/ tests.

[email protected], [email protected]

Change-Id: I0936f2daa4ab1cd91edeaca3eef9544eced5b443
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/106982
Commit-Queue: Konstantin Shcheglov <[email protected]>
Reviewed-by: Paul Berry <[email protected]>
Reviewed-by: Brian Wilkerson <[email protected]>

Author: scheglov

pkg/analyzer/lib/src/dart/analysis/library_analyzer.dart

@@ -21,6 +21,7 @@ import 'package:analyzer/src/dart/constant/utilities.dart';
 import 'package:analyzer/src/dart/element/element.dart';
 import 'package:analyzer/src/dart/element/handle.dart';
 import 'package:analyzer/src/dart/element/inheritance_manager2.dart';
+import 'package:analyzer/src/dart/resolver/flow_analysis_visitor.dart';
 import 'package:analyzer/src/dart/resolver/legacy_type_asserter.dart';
 import 'package:analyzer/src/error/codes.dart';
 import 'package:analyzer/src/error/inheritance_override.dart';
@@ -407,11 +408,17 @@ class LibraryAnalyzer {
         _context.typeSystem, _inheritance, errorReporter);
     inheritanceOverrideVerifier.verifyUnit(unit);
 
+    FlowAnalysisResult flowAnalysisResult;
+    if (unit.featureSet.isEnabled(Feature.non_nullable)) {
+      flowAnalysisResult = performFlowAnalysis(_context.typeSystem, unit);
+    }
+
     //
     // Use the ErrorVerifier to compute errors.
     //
     ErrorVerifier errorVerifier = new ErrorVerifier(
-        errorReporter, _libraryElement, _typeProvider, _inheritance, false);
+        errorReporter, _libraryElement, _typeProvider, _inheritance, false,
+        flowAnalysisResult: flowAnalysisResult);
     unit.accept(errorVerifier);
   }
 

pkg/analyzer/lib/src/dart/resolver/flow_analysis.dart

@@ -2,15 +2,17 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-/// Sets of variables that are potentially assigned in a statement.
+/// Sets of local variables that are potentially assigned in a statement.
+///
+/// These statements are loops, `switch`, and `try` statements.
 class AssignedVariables<Statement, Element> {
   final emptySet = Set<Element>();
 
-  /// Mapping from a [Statement] representing a loop to the set of variables
-  /// that are potentially assigned in that loop.
+  /// Mapping from a [Statement] to the set of local variables that are
+  /// potentially assigned in that statement.
   final Map<Statement, Set<Element>> _map = {};
 
-  /// The stack of nested nodes.
+  /// The stack of nested statements.
   final List<Set<Element>> _stack = [];
 
   AssignedVariables();
@@ -21,12 +23,12 @@ class AssignedVariables<Statement, Element> {
     return _map[statement] ?? emptySet;
   }
 
-  void beginLoop() {
+  void beginStatement() {
     var set = Set<Element>.identity();
     _stack.add(set);
   }
 
-  void endLoop(Statement node) {
+  void endStatement(Statement node) {
     _map[node] = _stack.removeLast();
   }
 
@@ -126,6 +128,17 @@ class FlowAnalysis<Statement, Expression, Element, Type> {
     _current = _current.add(variable, assigned: assigned);
   }
 
+  void booleanLiteral(Expression expression, bool value) {
+    _condition = expression;
+    if (value) {
+      _conditionTrue = _current;
+      _conditionFalse = _identity;
+    } else {
+      _conditionTrue = _identity;
+      _conditionFalse = _current;
+    }
+  }
+
   void conditional_elseBegin(Expression conditionalExpression,
       Expression thenExpression, bool isBool) {
     var afterThen = _current;
@@ -223,12 +236,6 @@ class FlowAnalysis<Statement, Expression, Element, Type> {
     _current = _join(falseCondition, breakState);
   }
 
-  void falseLiteral(Expression expression) {
-    _condition = expression;
-    _conditionTrue = _identity;
-    _conditionFalse = _current;
-  }
-
   void forEachStatement_bodyBegin(Set<Element> loopAssigned) {
     _stack.add(_current);
     _current = _current.removePromotedAll(loopAssigned);
@@ -494,12 +501,6 @@ class FlowAnalysis<Statement, Expression, Element, Type> {
     _stack.add(_current); // afterExpression
   }
 
-  void trueLiteral(Expression expression) {
-    _condition = expression;
-    _conditionTrue = _current;
-    _conditionFalse = _identity;
-  }
-
   void tryCatchStatement_bodyBegin() {
     _stack.add(_current);
     // Tail of the stack: beforeBody