Start to get instance fields working.

test/field_default_to_null.wren

@@ -0,0 +1,5 @@
+class Foo {
+  write { io.write(_field) }
+}
+
+Foo.new.write // expect: null

test/field_multiple.wren

@@ -0,0 +1,29 @@
+class Foo {
+  set(a, b, c, d, e) {
+    _a = a
+    _b = b
+    _c = c
+    _d = d
+    _e = e
+  }
+  write {
+    io.write(_a)
+    io.write(_b)
+    io.write(_c)
+    io.write(_d)
+    io.write(_e)
+  }
+}
+
+var foo = Foo.new
+foo.set(1, 2, 3, 4, 5)
+foo.write
+// expect: 1
+// expect: 2
+// expect: 3
+// expect: 4
+// expect: 5
+
+// TODO(bob): Inherited fields.
+// TODO(bob): Trying to get or set a field outside of a class.
+// TODO(bob): Fields in nested classes.

test/field_object_reference.wren

@@ -0,0 +1,37 @@
+// This test exists mainly to make sure the GC traces instance fields.
+class Node {
+  set(left, value, right) {
+    _left = left
+    _value = value
+    _right = right
+  }
+
+  write {
+    if (_left is Node) {
+      _left.write
+    }
+
+    io.write(_value)
+
+    if (_right is Node) {
+      _right.write
+    }
+  }
+}
+
+var a = Node.new
+a.set(null, "a", null)
+var b = Node.new
+b.set(null, "b", null)
+var c = Node.new
+c.set(a, "c", b)
+a = null
+b = null
+var d = Node.new
+d.set(c, "d", null)
+c = null
+d.write
+// expect: a
+// expect: c
+// expect: b
+// expect: d

test/field_use_before_set.wren

@@ -0,0 +1,9 @@
+class Foo {
+  write { io.write(_field) } // Compile a use of the field...
+  init { _field = "value" }  // ...before an assignment to it.
+}
+
+var foo = Foo.new
+// But invoke them in the right order.
+foo.init
+foo.write // expect: value

test/use_field_as_var.wren

@@ -0,0 +1 @@
+var _field = "value" // expect error