- Any prior use of BRAF inhibitors or MEK inhibitors. - Subjects who have received dacarbazine or paclitaxel before randomization will not be eligible to receive the same chemotherapy as study medication (i.e. a subject who received prior dacarbazine cannot receive dacarbazine on this trial and would thus receive paclitaxel if randomized to the control arm) - History of another malignancy. Exception: Subjects who have been disease-free for 3 years, or subjects with a history of completely resected non-melanoma skin cancer or successfully treated in situ carcinoma are eligible. Subjects with second malignancies that are indolent or definitively treated may be enrolled. - Known Human Immunodeficiency Virus (HIV), Hepatitis B Virus (HBV), or Hepatitis C Virus (HCV) infection (except for chronic or cleared HBV and HCV infection which will be allowed) - Brain metastases with the following exceptions that are ALL confirmed by the GSK Medical Monitor: All known lesions must be previously treated with surgery or stereotactic radiosurgery, and Brain lesion(s), if still present, must be confirmed stable (i.e. no increase in lesion size) for ≥90 days before randomization (must be documented with two consecutive MRI or CT scans using contrast), and asymptomatic with no corticosteroids requirement for ≥ 30 days before randomization, and no enzyme-inducing anticonvulsants for ≥30 days before randomization - History or evidence of cardiovascular risk including any of the following: • QTcB ≥ 480 msec. • History or evidence of current clinically significant uncontrolled arrhythmias. Exception: Subjects with controlled atrial fibrillation for >30 days before randomization are eligible • History of acute coronary syndromes (including myocardial infarction and unstable angina), coronary angioplasty, or stenting within 6 months before randomization. • History or evidence of current ≥ Class II congestive heart failure as defined by the New York Heart Association • History of interstitial lung disease or pneumonitis - History or current evidence/risk of retinal vein occlusion (RVO) or central serous retinopathy (CSR): • History of RVO or CSR, or predisposing factors to RVO or CSR (e.g. uncontrolled glaucoma or ocular hypertension, uncontrolled hypertension, uncontrolled diabetes mellitus, or history of hyperviscosity or hypercoagulability syndromes). • Visible retinal pathology as assessed by ophthalmic exam that is considered a risk factor for RVO or CSR such as: - Evidence of new optic disc cupping. - Intraocular pressure >21 mm Hg as measured by tonography