Disallow assigning null to non-nullable variables

pkg/analyzer/lib/src/task/strong/checker.dart

Index: pkg/analyzer/lib/src/task/strong/checker.dart
diff --git a/pkg/analyzer/lib/src/task/strong/checker.dart b/pkg/analyzer/lib/src/task/strong/checker.dart
index 21281719674209128269d53a0165ed8d8cab6fd2..c074b7938c7aec46eabdae9302f15523b0dd0659 100644
--- a/pkg/analyzer/lib/src/task/strong/checker.dart
+++ b/pkg/analyzer/lib/src/task/strong/checker.dart
@@ -163,6 +163,7 @@ class CodeChecker extends RecursiveAstVisitor {
       checkAssignment(expr.expression, type);
     } else {
       _checkDowncast(expr, type);
+      _checkNonNullAssignment(expr, type);
     }
   }
 
@@ -613,11 +614,10 @@ class CodeChecker extends RecursiveAstVisitor {
       // typing rules may have inferred a more precise type for the variable
       // based on the initializer.
     } else {
-      var dartType = getType(type);
       for (VariableDeclaration variable in node.variables) {
         var initializer = variable.initializer;
         if (initializer != null) {
-          checkAssignment(initializer, dartType);
+          checkAssignment(initializer, type.type);
         }
       }
     }
@@ -839,6 +839,14 @@ class CodeChecker extends RecursiveAstVisitor {
         [declElement.name]);
   }
 
+  void _checkNonNullAssignment(Expression expression, DartType type) {
+    var exprType = expression.staticType;
+    if (rules.isNonNullableType(type) && !rules.isNonNullableType(exprType)) {
+      _recordMessage(expression, StaticTypeWarningCode.INVALID_ASSIGNMENT,
+          [exprType, type]);
+    }
+  }
+
   void _checkReturnOrYield(Expression expression, AstNode node,
       {bool yieldStar: false}) {
     FunctionBody body = node.getAncestor((n) => n is FunctionBody);