Improve "why not promoted" context message for the case of writes.

The new message phrasing should flow better, especially in the analyzer CLI.

For the motivation for this change, please see
dart-lang/sdk#44904 (comment)

Bug: dart-lang/sdk#44898
Change-Id: I595492c64b42aff25cae5a8936c32aa8a218edd8
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/184601
Reviewed-by: Johnni Winther <[email protected]>
Commit-Queue: Paul Berry <[email protected]>

Author: stereotype441

pkg/_fe_analyzer_shared/lib/src/messages/codes_generated.dart

@@ -9665,15 +9665,13 @@ const MessageCode messageVarReturnType = const MessageCode("VarReturnType",
         r"""Try removing the keyword 'var', or replacing it with the name of the return type.""");
 
 // DO NOT EDIT. THIS FILE IS GENERATED. SEE TOP OF FILE.
-const Template<
-    Message Function(
-        String
-            name)> templateVariableCouldBeNullDueToWrite = const Template<
-        Message Function(String name)>(
-    messageTemplate:
-        r"""Variable '#name' could be null due to a write occurring here.""",
-    tipTemplate: r"""Try null checking the variable after the write.""",
-    withArguments: _withArgumentsVariableCouldBeNullDueToWrite);
+const Template<Message Function(String name)>
+    templateVariableCouldBeNullDueToWrite =
+    const Template<Message Function(String name)>(
+        messageTemplate:
+            r"""Variable '#name' could be null due to an intervening write.""",
+        tipTemplate: r"""Try null checking the variable after the write.""",
+        withArguments: _withArgumentsVariableCouldBeNullDueToWrite);
 
 // DO NOT EDIT. THIS FILE IS GENERATED. SEE TOP OF FILE.
 const Code<Message Function(String name)> codeVariableCouldBeNullDueToWrite =
@@ -9687,7 +9685,7 @@ Message _withArgumentsVariableCouldBeNullDueToWrite(String name) {
   name = demangleMixinApplicationName(name);
   return new Message(codeVariableCouldBeNullDueToWrite,
       message:
-          """Variable '${name}' could be null due to a write occurring here.""",
+          """Variable '${name}' could be null due to an intervening write.""",
       tip: """Try null checking the variable after the write.""",
       arguments: {'name': name});
 }

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

@@ -392,8 +392,8 @@ class _WhyNotPromotedVisitor
       String variableName, Expression writeExpression) {
     return DiagnosticMessageImpl(
         filePath: source.fullName,
-        message:
-            "Variable '$variableName' could be null due to a write occurring here.",
+        message: "Variable '$variableName' could be null due to an intervening "
+            "write.",
         offset: writeExpression.offset,
         length: writeExpression.length);
   }

pkg/front_end/messages.yaml

@@ -4589,7 +4589,7 @@ MultipleVarianceModifiers:
   analyzerCode: ParserErrorCode.MULTIPLE_VARIANCE_MODIFIERS
 
 VariableCouldBeNullDueToWrite:
-  template: "Variable '#name' could be null due to a write occurring here."
+  template: "Variable '#name' could be null due to an intervening write."
   tip: "Try null checking the variable after the write."
 
 FieldNotPromoted:

pkg/front_end/test/spell_checking_list_messages.txt

@@ -36,6 +36,7 @@ futureor
 h
 https
 interop
+intervening
 js_util
 libraries.json
 list.filled