Show parent pubspecs when a pubspec could not be found.

lib/src/package.dart

@@ -140,6 +140,9 @@ class Package {
   /// This mechanism can be used to avoid loading pubspecs twice. It can also be
   /// used to override a pubspec in memory for trying out an alternative
   /// resolution.
+  ///
+  /// [referringPubspec] is the path to the pubspec that includes this one in
+  /// the workspace.
   factory Package.load(
     String dir,
     SourceRegistry sources, {
@@ -162,20 +165,29 @@ class Package {
       withPubspecOverrides: withPubspecOverrides,
       expectedName: expectedName,
     );
-    final workspacePackages = pubspec.workspace
-        .map(
-          (e) => Package.load(
-            p.join(dir, e),
+
+    final workspacePackages = pubspec.workspace.map(
+      (workspacePath) {
+        try {
+          return Package.load(
+            p.join(dir, workspacePath),
             sources,
             loadPubspec: loadPubspec,
             withPubspecOverrides: withPubspecOverrides,
-          ),
-        )
-        .toList();
+          );
+        } on FileException catch (e) {
+          throw FileException(
+            '${e.message}\n'
+            'That was included in the workspace of ${p.join(dir, 'pubspec.yaml')}.',
+            e.path,
+          );
+        }
+      },
+    ).toList();
     for (final package in workspacePackages) {
       if (package.pubspec.resolution != Resolution.workspace) {
         fail('''
-${package.pubspecPath} is inluded in the workspace from ${p.join(dir, 'pubspec.yaml')}, but does not have `resolution: workspace`.
+${package.pubspecPath} is included in the workspace from ${p.join(dir, 'pubspec.yaml')}, but does not have `resolution: workspace`.
 
 See $workspacesDocUrl for more information.
 ''');

test/workspace_test.dart

@@ -355,6 +355,38 @@ void main() {
     );
   });
 
+  test('reports missing pubspec.yaml of workspace member correctly', () async {
+    await dir(appPath, [
+      libPubspec(
+        'myapp',
+        '1.2.3',
+        extras: {
+          'workspace': ['a'],
+        },
+        sdk: '^3.7.0',
+      ),
+      dir('a', [
+        libPubspec(
+          'a',
+          '1.0.0',
+          resolutionWorkspace: true,
+          extras: {
+            'workspace': ['b'], // Doesn't exist.
+          },
+        ),
+      ]),
+    ]).create();
+    await pubGet(
+      environment: {'_PUB_TEST_SDK_VERSION': '3.7.0'},
+      error: contains(
+        'Could not find a file named "pubspec.yaml" in "${p.join(sandbox, appPath, 'a', 'b')}".\n'
+        'That was included in the workspace of ${p.join('.', 'a', 'pubspec.yaml')}.\n'
+        'That was included in the workspace of ${p.join('.', 'pubspec.yaml')}.',
+      ),
+      exitCode: NO_INPUT,
+    );
+  });
+
   test('`pub deps` lists dependencies for all members of workspace', () async {
     final server = await servePackages();
     server.serve(