Finish implementation of GetReferenceDirectives in VB and enable test...

(fixes DevDiv 790235)

src/Compilers/VisualBasic/Portable/Syntax/CompilationUnitSyntax.vb

@@ -19,10 +19,9 @@ Namespace Microsoft.CodeAnalysis.VisualBasic.Syntax
         End Function
 
         Friend Function GetReferenceDirectives(filter As Func(Of ReferenceDirectiveTriviaSyntax, Boolean)) As IList(Of ReferenceDirectiveTriviaSyntax)
-            ' TODO: tomat
-            ' enable test ReferenceManagerReuse_WithSyntaxTrees when implemented
-            ' Return SyntaxNode.GetDirectives(Me.GetFirstToken(includeZeroWidth:=True), filter)
-            Return SpecializedCollections.EmptyList(Of ReferenceDirectiveTriviaSyntax)()
+            ' #r directives are always on the first token of the compilation unit.
+            Dim firstToken = CType(Me.GetFirstToken(includeZeroWidth:=True), SyntaxNodeOrToken)
+            Return firstToken.GetDirectives(Of ReferenceDirectiveTriviaSyntax)(filter)
         End Function
     End Class
 End Namespace

src/Compilers/VisualBasic/Test/Semantic/Compilation/CompilationAPITests.vb

@@ -1485,7 +1485,7 @@ End Class
             Assert.False(c3.ReferenceManagerEquals(c2))
         End Sub
 
-        <Fact(Skip:="790235")>
+        <Fact>
         Public Sub ReferenceManagerReuse_WithSyntaxTrees()
             Dim ta = Parse("Imports System")
             Dim tb = Parse("Imports System", options:=TestOptions.Script)
@@ -1536,12 +1536,6 @@ End Class
 
             Dim ars = arc.ReplaceSyntaxTree(tc, ts)
             Assert.True(arc.ReferenceManagerEquals(ars))
-
-            Dim ar3 = arc.ReplaceSyntaxTree(tc, ta)
-            Assert.True(arc.ReferenceManagerEquals(ar3))
-
-            Dim as1 = ars.ReplaceSyntaxTree(tr, ts)
-            Assert.False(ars.ReferenceManagerEquals(as1))
         End Sub
 
         Private Class EvolvingTestReference