Make SyntaxGenerator.MethodDeclaration properly handle partial methods

Fixes #1604.
Closes #1605.

src/Workspaces/CSharp/Portable/CodeGeneration/CSharpSyntaxGenerator.cs

@@ -147,7 +147,7 @@ private SyntaxTokenList GetParameterModifiers(RefKind refKind)
             parameters = this.ClearTrivia(parameters);
             returnType = this.ClearTrivia(returnType);
 
-            bool hasBody = !modifiers.IsAbstract;
+            bool hasBody = !modifiers.IsAbstract && (!modifiers.IsPartial || statements != null);
 
             return SyntaxFactory.MethodDeclaration(
                 default(SyntaxList<AttributeListSyntax>),

src/Workspaces/CSharpTest/CodeGeneration/SyntaxGeneratorTests.cs

@@ -614,6 +614,14 @@ public void TestMethodDeclarations()
             VerifySyntax<MethodDeclarationSyntax>(
                 _g.MethodDeclaration("m", returnType: _g.IdentifierName("x"), accessibility: Accessibility.Public, modifiers: DeclarationModifiers.Abstract),
                 "public abstract x m();");
+
+            VerifySyntax<MethodDeclarationSyntax>(
+                _g.MethodDeclaration("m", modifiers: DeclarationModifiers.Partial),
+                "partial void m();");
+
+            VerifySyntax<MethodDeclarationSyntax>(
+                _g.MethodDeclaration("m", modifiers: DeclarationModifiers.Partial, statements: new[] { _g.IdentifierName("y") }),
+                "partial void m()\r\n{\r\n    y;\r\n}");
         }
 
         [Fact]

src/Workspaces/VisualBasicTest/CodeGeneration/SyntaxGeneratorTests.vb

@@ -682,6 +682,11 @@ End Function</x>.Value)
             VerifySyntax(Of MethodStatementSyntax)(
                 _g.MethodDeclaration("m", returnType:=_g.IdentifierName("x"), accessibility:=Accessibility.Public, modifiers:=DeclarationModifiers.Abstract),
 <x>Public MustInherit Function m() As x</x>.Value)
+
+            VerifySyntax(Of MethodBlockSyntax)(
+                _g.MethodDeclaration("m", accessibility:=Accessibility.Private, modifiers:=DeclarationModifiers.Partial),
+<x>Private Partial Sub m()
+End Sub</x>.Value)
         End Sub
 
         <Fact>