Improve warnings related to incorrect 'part of' declarations.

pkg/front_end/lib/src/fasta/source/source_library_builder.dart

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // 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.
 
 library fasta.source_library_builder;
 
 import 'package:front_end/src/scanner/token.dart' show Token;
 
 import 'package:kernel/ast.dart' show ProcedureKind;
 
@@ skipping 48 matching lines @@
   final List<List> implementationBuilders = <List<List>>[];
 
   /// Indicates whether type inference (and type promotion) should be disabled
   /// for this library.
   final bool disableTypeInference;
 
   String name;
 
   String partOfName;
 
-  Uri partOfUri;
+  String partOfUri;
 
   List<MetadataBuilder> metadata;
 
   /// The current declaration that is being built. When we start parsing a
   /// declaration (class, method, and so on), we don't have enough information
   /// to create a builder and this object records its members and types until,
   /// for example, [addClass] is called.
   DeclarationBuilder<T> currentDeclaration;
 
   bool canAddImplementationBuilders = false;
@@ skipping 90 matching lines @@
       if (uri.scheme != "package") {
         newFileUri = fileUri.resolve(path);
       }
     }
     parts.add(loader.read(resolvedUri, charOffset,
         fileUri: newFileUri, accessor: this));
   }
 
   void addPartOf(List<MetadataBuilder> metadata, String name, String uri) {
     partOfName = name;
-    partOfUri = uri == null ? null : this.uri.resolve(uri);
+    partOfUri = uri;
   }
 
   void addClass(
       List<MetadataBuilder> metadata,
       int modifiers,
       String name,
       List<TypeVariableBuilder> typeVariables,
       T supertype,
       List<T> interfaces,
       int charOffset);
@@ skipping 215 matching lines @@
       } else if (seenParts.add(part.fileUri)) {
         includePart(part);
       } else {
         addCompileTimeError(
             -1, "Can't use '${part.fileUri}' as a part more than once.");
       }
     }
   }
 
   void includePart(SourceLibraryBuilder<T, R> part) {
-    if (name != null) {
-      if (!part.isPart) {
-        addCompileTimeError(
-            -1,
-            "Can't use ${part.fileUri} as a part, because it has no 'part of'"
-            " declaration.");
-        parts.remove(part);
-        return;
-      }
-      if (part.partOfName != name && part.partOfUri != uri) {
-        String partName = part.partOfName ?? "${part.partOfUri}";
-        String myName = name == null ? "'$uri'" : "'${name}' ($uri)";
+    if (part.partOfUri != null) {
+      if (uri.resolve(part.partOfUri) != uri) {
+        // This is a warning, but the part is still included.
         addWarning(
             -1,
-            "Using '${part.fileUri}' as part of '$myName' but its 'part of'"
-            " declaration says '$partName'.");
-        // The part is still included.
+            "Using '${part.relativeFileUri}' as part of '$uri' but its "
+            "'part of' declaration says '${part.partOfUri}'.");
+        if (uri.scheme == "dart" && relativeFileUri.endsWith(part.partOfUri)) {
+          addWarning(-1, "See https://github.com/dart-lang/sdk/issues/30072.");
+        }
       }
+    } else if (part.partOfName != null) {
+      if (name != null) {
+        if (part.partOfName != name) {
+          // This is a warning, but the part is still included.
+          addWarning(
+              -1,
+              "Using '${part.relativeFileUri}' as part of '$name' but its "
+              "'part of' declaration says '${part.partOfName}'.");
+        }
+      } else {
+        // This is a warning, but the part is still included.
+        addWarning(
+            -1,
+            "Using '${part.relativeFileUri}' as part of '${relativeFileUri}' "
+            "but its 'part of' declaration says '${part.partOfName}'.\n"
+            "Try changing the 'part of' declaration to use a relative "
+            "file name.");
+      }
+    } else if (name != null) {
+      // This is an error, and the part isn't included.
+      assert(!part.isPart);
+      addCompileTimeError(
+          -1,
+          "Can't use ${part.fileUri} as a part, because it has no 'part of'"
+          " declaration.");
+      parts.remove(part);
+      return;
     }
     part.forEach((String name, Builder builder) {
       if (builder.next != null) {
         assert(builder.next.next == null);
         addBuilder(name, builder.next, builder.next.charOffset);
       }
       addBuilder(name, builder, builder.charOffset);
     });
     types.addAll(part.types);
     constructorReferences.addAll(part.constructorReferences);
@@ skipping 164 matching lines @@
   /// synthesize type variables on the factory matching the class'.
   void addFactoryDeclaration(
       ProcedureBuilder procedure, DeclarationBuilder<T> factoryDeclaration) {
     factoryDeclarations[procedure] = factoryDeclaration;
   }
 
   Scope toScope(Scope parent) {
     return new Scope(members, setters, parent, isModifiable: false);
   }
 }